Fuel compositions and method and uses relating thereto

Abstract

Additive for a fuel or lubricating composition comprising a quaternary ammonium salt of an itaconic based compound. Improving the performance of an engine using a quaternary ammonium salt of an itacon

Description

The present invention relates to additives useful in fuel and lubricating compositions, to compositions comprising such additives and to methods and uses relating thereto. Additives are commonly added to fuels and lubricating oils for various purposes, for example to improve stability on storage, to improve flowability through pipelines or to improve the performance of an engine. Detergent additives are commonly added to fuel and lubricating oil compositions to improve the performance of the engines using these compositions. The presence of detergents prevents the fouling of the moving parts of the engine. Without these additives the performance of the engine would diminish over time and the engine would eventually fail. Many different types of compounds have been used as detergents. However as engines change and develop and more and more requirements are imposed for environmental reasons, there is a continuing need to provide new detergent additives for fuel and lubricating oil compositions. The present inventors have surprisingly found that quaternary ammonium salts of itaconic acid derived compounds are effective detergents. According to a first aspect of the present invention there is provided an additive composition for a fuel or lubricating oil comprising as an additive at least one quaternary ammonium salt comprising cations Q+ and anions An-, wherein Q+ is a quaternary ammonium cation; An- is an itaconic acid based anion; and n is at least 1. The present invention relates to an additive which is a quaternary ammonium salt. This may be referred to herein as a quaternary ammonium compound, a quaternary ammonium salt or a quaternary ammonium salt additive. It may also be referred to as the additive of the invention. The present invention relates to the use of at least one quaternary ammonium salt as an additive. In some embodiments the additive incudes a single (one) quaternary ammonium salt. In some embodiments a mixture of two or more quaternary ammonium salts may be used. Such mixtures may result from the synthesis of the quaternary ammonium salts from mixtures of starting materials. Alternatively or additionally mixtures maybe obtained by combining fully synthesised compounds. All such embodiments are within the scope of the invention and references herein to “the” quaternary ammonium salt or “a” quaternary ammonium salt include embodiments in which mixtures are used. The quaternary ammonium salt comprises quaternary ammonium cations Q+ and itaconic acid based anions An-. In some embodiments the quaternary ammonium salt may include further cations, in addition to the cations Q+ and / or further anions in addition to the anions An-. In preferred embodiments no such further cations or anions are present and the quaternary ammonium salt has the formula (Q+)nAn-. n is at least 1. Preferably n is more than one and An- is a polyvalent anion. The quaternary ammonium salts are suitably prepared by reaction of a compound including a tertiary amino group and a quaternising agent. In some embodiments reaction of the compound including a tertiary amino group and a quaternising agent may directly provide a quaternary ammonium salt comprising cations Q+ and anions An-. In some embodiments the reaction of the compound including a tertiary amino group and the quaternising agent may provide a different compound which undergoes an anion exchange reaction to provide a quaternary ammonium salt of the invention comprising cation Q+ and anions An-. In some embodiments the quaternary ammonium compounds of the present invention may be prepared by reaction of a compound including a tertiary amino group with a quaternising agent selected from an ester of a carboxylic acid, dialkyl sulfates, benzyl halides, hydrocarbyl substituted carbonates, alkyl halides or mixtures thereof, followed by an anion exchange reaction. Suitable methods of carrying out such quaternisation and ion exchange reactions will be known to the person skilled in the art. In some embodiments the quaternising agent is an acid activated quaternising agent, preferably an epoxide and this is reacted in the presence of an acid. When this acid is itaconic acid based the quaternary ammonium salt of the invention may be directly obtained. In some embodiments the quaternary ammonium salt is provided by the reaction of: (a) a compound including a tertiary amino group; with (b) a quaternising agent; in the presence of (c) itaconic acid or an ester and / or polymer thereof. In such embodiments the quaternising agent is preferably an epoxide. In some embodiments the quaternary ammonium salt is provided by the reaction of (a) a compound including a tertiary amino group; with (b) a quaternising agent; followed by an anion exchange reaction with (c) itaconic acid or an ester and / or polymer thereof. Thus in some embodiments the compound including a tertiary amino group (a) is directly reacted with the quaternising agent in the presence of itaconic acid or an ester and / or polymer thereof. In other embodiments the compound including a tertiary amino group is first reacted with a different quaternising agent to provide a different quaternary ammonium salt having a different anion. This is followed by an anion exchange reaction by reaction with itaconic acid or an ester and / or polymer thereof. In some embodiments in which the quaternising agent is an epoxide this may be reacted with the compound including a tertiary amino group in the presence of a different acid and then subsequently reacted with itaconic acid or an ester and / or polymer thereof via a subsequent anion exchange reaction. In preferred embodiments when an epoxide quaternising agent is used this is preferably reacted with the compound including a tertiary amino group in the presence of itaconic acid or an ester and / or polymer thereof. Component (a) comprises a compound including a tertiary amino group. This tertiary amino group suitably reacts with the quaternising agent to provide the cation Q+. Component (a) may comprise any compound which includes at least one tertiary amino group. The compound may include more than one tertiary amino group. Preferably the compound does not include any primary or secondary amino groups. The compound including a tertiary amino group (a) may be selected from: (i) the reaction product of a hydrocarbyl-substituted acylating agent and a compound having at least one tertiary amine group and a primary amine, secondary amine or alcohol group; (ii) a Mannich reaction product comprising a tertiary amine group; (iii) a polyalkylene substituted amine having at least one tertiary amino group; (iv) a tertiary amine of formula R5R6R7N, wherein each of R5, R6 and R7 is independently an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group; (v) a cyclic tertiary amine; and (vi) a polyetheramine compound. In some embodiments the compound including the tertiary amino group is (i) the reaction product of a hydrocarbyl-substituted acylating agent and a compound comprising at least one tertiary amine group and a primary amine, secondary amine or alcohol group. Suitable hydrocarbyl substituted acylating agents for use herein include fatty acids, i.e. compounds of formula RCOOH in which R is an alkyl or alkenyl group having 6 to 36 carbon atoms, preferably 8 to 30 carbon atoms or 12 to 24 carbon atoms. One preferred fatty acid is oleic acid. The hydrocarbyl substituted acylating agent may be based on a hydrocarbyl substituted mono-di- or polycarboxylic acid or a reactive equivalent thereof. In some preferred embodiments the hydrocarbyl substituted acylating agent is a hydrocarbyl substituted succinic acid compound, for example a hydrocarbyl substituted succinic acid or succinic anhydride. The hydrocarbyl substituent preferably comprises at least 10, more preferably at least 12, for example 30 or 50 carbon atoms. It may comprise up to about 200 carbon atoms. Preferably the hydrocarbyl substituent has a number average molecular weight (Mn) of between 170 to 2800, for example from 250 to 1500, preferably from 450 to 1500 and more preferably 450 to 1100. An Mn of 700 to 1300 is especially preferred. As used herein, the term "hydrocarbyl substituent" 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. In this specification, unless otherwise stated references to optionally substituted alkyl groups may include aryl-substituted alkyl groups and references to optionally-substituted aryl groups may include alkyl-substituted or alkenyl-substituted aryl groups. In some preferred embodiments component (i) comprises the reaction product of a hydrocarbyl-substituted succinic acid derivative and an alcohol or amine also including a tertiary amine group. Preferred hydrocarbyl-based substituents are poly-(isobutene)s. Such compounds are known in the art. Thus in some especially preferred embodiments the hydrocarbyl substituted acylating agent is a polyisobutenyl substituted succinic acid or succinic anhydride. Polyisobutenyl substituted succinic anhydrides are especially preferred. The preparation of polyisobutenyl substituted succinic anhydrides (PIBSA) is documented in the art. Suitable processes will be known to the person skilled in the art. Conventional polyisobutenes and so-called "highly-reactive" polyisobutenes are suitable for use in the invention. Highly reactive polyisobutenes in this context are defined as polyisobutenes wherein at least 50%, preferably 70% or more, of the terminal olefinic double bonds are of the vinylidene type as described in EP0565285. Particularly preferred polyisobutenes are those having more than 80 mol% and up to 100% of terminal vinylidene groups such as those described in EP1344785. Other preferred hydrocarbyl groups include those having an internal olefin for example as described in the applicant’s published application WO2007 / 015080. An internal olefin as used herein means any olefin containing predominantly a non-alpha double bond, that is a beta or higher olefin. Preferably such materials are substantially completely beta or higher olefins, for example containing less than 10% by weight alpha olefin, more preferably less than 5% by weight or less than 2% by weight. Typical internal olefins include Neodene 1518 IO available from Shell. Internal olefins are sometimes known as isomerised olefins and can be prepared from alpha olefins by a process of isomerisation known in the art, or are available from other sources. The fact that they are also known as internal olefins reflects that they do not necessarily have to be prepared by isomerisation. In some preferred embodiments the compound including the tertiary amino group is the reaction product of an alcohol or amine including a tertiary amino group and an optionally substituted succinic acid or anhydride thereof of formula (1) or (2): wherein R1 is an optionally substituted hydrocarbyl group. Preferably R1 is an optionally substituted alkyl or alkenyl group. R1 may be substituted with one or more groups selected from halo (e.g. chloro, fluoro or bromo), nitro, hydroxy, mercapto, sulfoxy, amino, nitryl, acyl, carboxy, alkyl (e.g. Ci to C4 alkyl), alkoxyl (e.g. Ci to C4 alkoxy), amido, keto, sulfoxy and cyano. Preferably R1 is an unsubstituted alkyl or alkenyl group. The substituted succinic acid or anhydrides may suitably be prepared by reacting maleic anhydride with an alkene. In some preferred embodiments R1 has a number average molecular weight of from 100 to 5000, preferably from 300 to 4000, suitably from 450 to 2500, for example from 450 to 2000 or from 450 to 1500. In especially preferred embodiments the compound including a tertiary amino group is the reaction product of a hydrocarbyl substituted succinic acid or an anhydride thereof substituted with a polyisobutenyl group having a number average molecular weight of 450 to 1500 and an alcohol or amine which further includes a tertiary amino group. In some embodiments the substituted succinic acid or anhydride thereof may comprise a mixture of compounds including groups R1 of different lengths. In such embodiments any reference to the molecular weight of the group R1 relates to the number average molecular weight of all of that group for all compounds in the composition. In preferred embodiments R1 is a polyisobutenyl group, preferably having a number average molecular weight of from 100 to 5000, preferably from 200 to 2400, suitably from 450 to 1500. In some embodiments R1 is an optionally substituted Ci to C500 alkyl or alkenyl group, for example a Cs to C40 alkyl or alkenyl group, suitably C16 to C36 alkyl or alkenyl group. In some embodiments the compound including a tertiary amino group is the reaction product of a succinic acid or anhydride having a C10 to C30, preferably a C20 to C24 alkyl or alkenyl group and an amine or alcohol which further includes a tertiary amino group. Preferred hydrocarbyl substituted acylating agents for use herein are polyisobutenyl substituted succinic anhydrides or PIBSAs. Especially preferred PIBSAs are those having a PIB molecular weight (Mn) of from 300 to 2800, preferably from 450 to 2300, more preferably from 500 to 1300. The hydrocarbyl substituted succinic acid derived acylating agent is suitably prepared by reacting maleic anhydride with an alkene, for example a polyisobutene. The product obtained (such as a PIBSA) still includes a double bond. The maleic anhydride is present in the resultant molecule as a succinic acid moiety. This initial product is a monomaleated PIBSA. The monomaleated PIBSA may have the structure (3) or (4): (3) (4) The double bond in the monomaleated product can react with a further molecule of maleic anhydride to form a bismaleated PIBSA having the structure (C) or (D): (5) (6) Thus it is possible to provide a hydrocarbyl group which is substituted with more than one succinic acid moiety. The person skilled in the art will appreciate that in the preparation of PIBSAs from the reaction of PIB with maleic acid (MA), a mixture of products will result. Typically reaction mixtures include some unreacted PIB, some PIBSA from the reaction of PIB with one MA (monomaleated PIBSA) and some PIBSA from the reaction of PIB with two MA (bismaleated PIBSA). The fraction of bismaleated product as a proportion of the total PIBSA product may be referred to as the bismaleation level (BML). Suitable PIBSAs for use in preparing component (i) may have a BML of up to 90%, suitably up to 70%, for example 1 to 50% or 2 to 30%. The skilled person will appreciate that the hydrocarbyl substituted succinic acid derived acylating agents used in the invention typically comprise mixtures of compounds, for example mixtures of monomaleated and bismaleated PIBSAs. The PIBSAs may be defined in terms of their level of bismaleation. One way in which this may be determined is by calculating the average number of succinic acid moieties per molecule of acylating agent. A monomaleated PIBSA has one succinic acid moiety per molecule. A bismaleated PIBSA has two succinic acid moieties per molecule. A mixture comprising monomaleated PIBSA and bismaleated PIBSA in a 1:1 molar ratio would comprise an average of 1.5 succinic acid moieties per molecule of PIBSA. The average number of succinic acid moieties per molecule of acylating agent is sometimes referred to in the art as “P value”. Suitably the or each quaternary ammonium compound is prepared from a hydrocarbyl substituted succinic acid derived acylating agent comprising on average from 1 to 2 succinic acid moieties per molecule, for example an average from 1.05 to 1:5 or from 1.1 to 1.3 succinic acid moieties per molecule. The hydrocarbyl substituted acylating agent is reacted with a compound able to react with said acylating agent and which includes a tertiary amine group. Examples of suitable compounds able to react with the hydrocarbyl substituted succinic acid derived acylating agent and which include a tertiary amine group can include but are not limited to: N,N-dimethylaminopropylamine, N,N-diethylaminopropylamine, N,N-dimethylamino ethylamine. The nitrogen or oxygen containing compounds capable of condensing with the acylating agent and further having a tertiary amino group can further include amino alkyl substituted heterocyclic compounds such as 1-(3-aminopropyl)imidazole and 4-(3-aminopropyl)morpholine, 1-(2-aminoethyl)piperidine, 3,3-diamino-N-methyldipropylamine, and 3'3-aminobis(N,N-dimethylpropylamine). Other types of nitrogen or oxygen containing compounds capable of condensing with the acylating agent and having a tertiary amino group include alkanolamines including but not limited to triethanolamine, trimethanolamine, N,N-dimethylaminopropanol, N,N-dimethylaminoethanol, N,N-diethylaminopropanol, N,N-diethylaminoethanol, N,N-diethylaminobutanol, N,N,N-tris(hydroxyethyl)amine, N,N,N-tris(hydroxymethyl)amine, N,N,N-tris(aminoethyl)amine, N,N-dibutylaminopropylamine and N,N,N'-trimethyl-N'-hydroxyethyl-bisaminoethylether; N,N-bis(3-dimethylaminopropyl)-N-isopropanolamine; N-(3-dimethylaminopropyl)-N,N-diisopropanolamine; N'-(3-(dimethylamino) propyl)-N,N-dimethyl 1,3-propanediamine; 2-(2-dimethylaminoethoxy)ethanol, N,N,N'-trimethylaminoethylethanolamine and 3-(2-(dimethylamino)ethoxy) propylamine. Preferred compounds including a tertiary amino group of type (i) are formed by the reaction of a hydrocarbyl-substituted acylating agent and an amine of formula (7) or (8): R2 N--X--NHR4 R3 (7) [O(CR42)m]nOH R3 (8) wherein R2 and R3 are the same or different alkyl groups having from 1 to 36 carbon atoms; X is an alkylene group having from 1 to 20 carbon atoms and wherein the alkylene group may be optionally interrupted with one or more heteroatoms; n is from 0 to 20; m is from 1 to 5; and R4 is hydrogen or a Ci to C36 alkyl group. The compound including a tertiary amino group may be formed by the reaction of a hydrocarbyl substituted acylating agent and an amine of formula (7) or (8). When a compound of formula (7) is used, R4 is preferably hydrogen or a Ci to C16 alkyl group, preferably a Ci to C10 alkyl group, more preferably a Ci to Ce alkyl group. When R4 is alkyl it may be straight chained or branched. It may be substituted for example with a hydroxy or alkoxy substituent. Preferably R4 is not a substituted alkyl group. More preferably R4 is selected from hydrogen, methyl, ethyl, propyl, butyl and isomers thereof. Most preferably R4 is hydrogen. When a compound of formula (8) is used, each R4 is preferably hydrogen or a Ci to Ce alkyl group. More preferably each R4 is selected from hydrogen, methyl, ethyl, propyl, butyl and isomers thereof. Most preferably each R4 is hydrogen or methyl. When a compound of formula (8) is used, m is preferably 2 or 3, most preferably 2; n is preferably from 0 to 15, preferably 0 to 10, more preferably from 0 to 5. Most preferably n is 0 and the compound of formula (8) is an alcohol. In some preferred embodiments the hydrocarbyl substituted acylating agent is reacted with a diamine compound of formula (7). R2 and R3 are the same or different alkyl, alkenyl or aryl groups having from 1 to 22 carbon atoms. In some embodiments R2 and R3 may be joined together to form a ring structure, for example a piperidine or imidazole moiety. R2 and R3 may be branched alkyl or alkenyl groups. Each may be substituted, for example with a hydroxy or alkoxy substituent. R2 and R3 may each independently be a Ci to C16 alkyl group, preferably a Ci to C10 alkyl group. R2and R3 may independently be methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, or an isomer of any of these. Preferably R2 and R3 is each independently Ci to C4 alkyl. Preferably R2 is methyl. Preferably R3 is methyl. X is an alkylene group having from 1 to 20 carbon atoms. In preferred embodiments the alkylene group X may be straight chained or branched. The alkylene group may include a cyclic structure therein. It may be optionally substituted, for example with a hydroxy or alkoxy substituent. The alkylene group X may be optionally interrupted with one or more heteroatoms, for example O, NH or N-alkyl. X is preferably an alkylene group having 1 to 16 carbon atoms, preferably 1 to 12 carbon atoms, more preferably 1 to 8 carbon atoms, for example 2 to 6 carbon atoms or 2 to 5 carbon atoms. Most preferably X is an ethylene, propylene or butylene group, especially a propylene group. Examples of compounds of formula (7) 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- dimethy Ipropy lamine), N'-(3-(dimethylamino)propyl)-N,N-dimethyl-1,3-propanediamine, 3-(2-(dimethylamino)ethoxy)propylamine or combinations thereof. In some preferred embodiments the compound of formula (7) is selected from from N,N-dimethyl-1,3-diaminopropane, N,N-diethyl-1,3- diaminopropane, N,N-dimethylethylenediamine, N,N-diethylethylenediamine, N,N-dibutylethylenediamine, or combinations thereof. An especially preferred compound of formula (7) is dimethylaminopropylamine. Examples of compounds of formula (8) suitable for use herein include alkanolamines including but not limited to triethanolamine, N,N-dimethylaminopropanol, N,N-diethylaminopropanol, N,N-diethylaminobutanol, triisopropanolamine, 1-[2-hydroxyethyl]piperidine, 2-[2-(dimethylamine)ethoxy]-ethanol, N-ethyldiethanolamine, N-methyldiethanolamine, N-butyldiethanolamine, N,N-diethylaminoethanol, N,N-dimethyl amino- ethanol, 2-dimethylamino-2-methyl-1-propanol; trimethanolamine, N,N,N-tris(hydroxymethyl)amine, N,N,N- tris(aminoethyl)amine, N,N-bis(3-dimethylaminopropyl)-N-isopropanolamine and N-(3-dimethylaminopropyl)-N,N-diisopropanolamine. In some preferred embodiments the compound of formula (8) is selected from N, N-dimethylaminopropanol, triisopropanolamine, 1-[2-hydroxyethyl]piperidine, 2-[2-(dimethylamine)ethoxy]-ethanol, N-ethyldiethanolamine, N-methyldiethanolamine, N-butyldiethanolamine, N,N-diethylaminoethanol, N,N-dimethylaminoethanol, 2-dimethylamino-2-methyl-1-propanol, or combinations thereof. An especially preferred compound of formula (8) is dimethylaminopropanol. Some preferred acylating agents for use in the preparation of the compound including a tertiary amino group are polyisobutene-substituted succinic acids or succinic anhydrides. When a compound of formula (8) is reacted with a succinic acylating agent the resulting product is a succinic ester. When a succinic acylating agent is reacted with a compound of formula (7) in which R4 is hydrogen the resulting product may be a succinimide or a succinamide. When a succinic acylating agent is reacted with a compound of formula (7) in which R4 is not hydrogen the resulting product is an amide. Thus in some embodiments component (i) may be the reaction product of a succinic acid derivative and an amine or alcohol which is an ester or an amide and which also includes a further unreacted carboxylic acid group. This further carboxylic acid functional group can react with another amine or alcohol when an excess is used to form a diester or the diamide. For the avoidance of doubt, succinic esters include the monoester compounds having the general formula (9) and the diester compounds having the general formula (10); succinimides have the general formula (11); and succinamides include the monoamide compounds having the general formula (12) and the diamide compounds having have the general formula (13): It will be appreciated that isomers of (9) and (12) may be formed in which the other carboxylic acid group is esterified / amidated. The groups R shown in figures (9) to (13) include a tertiary amino group. This group may be quaternised by reaction with a quaternising agent. For compounds of formula (10) or (13) which include two tertiary amino groups, each of these may be reacted with a quaternising agent. In some embodiments mixtures of compounds having formula (9) and (10) or mixtures containing compounds (11) and / or (12) and / or (13) may be used. In preferred embodiments a succinic acid derivative is reacted with an amine (also including a tertiary amine group) under conditions to form a succinimide. In some embodiments the acid / anhydride and the alcohol / amine are reacted in a molar ratio of from 10:1 to 1:10, preferably from 5:1 to 1:5, more preferably from 2:1 to 1:2, for example from 1.5:1 to 1:1.5. Preferably the acid / anhydride and the alcohol / amine are reacted in an approximately 1:1 molar ratio, for example from 1.2:1 to 1:1.2. In some embodiments the compound including a tertiary amino group comprises a compound prepared from the reaction product of an optionally substituted succinic acid or anhydride thereof, preferably a hydrocarbyl substituted succinic acid or anhydride thereof, and an alcohol or amine selected from dimethylaminopropanol, dimethylaminopropylamine, N,N-diethyl-1,3-diaminopropane, N,N-dimethylethylenediamine, N,N-diethylethylenediamine, N,N-dibutylethylenediamine, 3-(2-(dimethylamino)ethoxy)propylamine or combinations thereof. In some especially preferred embodiments the compound including a tertiary amino group is prepared from an amine which includes a tertiary amino group (for example dimethylamino propylamine) and a polyisobutylene-substituted succinic anhydride. The number average molecular weight of the polyisobutylene substituent is preferably from 450 to 1300, more preferably from 900 to 1100. In some embodiments the compound including a tertiary amine group may be the reaction product of a fatty acid (for example oleic acid) and a compound of formula (7) or (8) (for example dimethylaminopropyl amine). In some embodiments the compound including a tertiary amino group may be (ii) a Mannich reaction product including a tertiary amine. The preparation of quaternary ammonium salts formed from nitrogen-containing species including component (ii) is described in US2008 / 0052985. The Mannich reaction product having a tertiary amino group is prepared from the reaction of a hydrocarbyl-substituted phenol, an aldehyde and an amine. The hydrocarbyl substituent of the hydrocarbyl substituted phenol can have 6 to 400 carbon atoms, suitably 30 to 180 carbon atoms, for example 10 or 40 to 110 carbon atoms. This hydrocarbyl substituent can be derived from an olefin or a polyolefin. Useful olefins include alpha-olefins, such as 1-decene, which are commercially available. The polyolefins which can form the hydrocarbyl substituent can be prepared by polymerizing olefin monomers by well known polymerization methods and are also commercially available. Some preferred polyolefins include polyisobutylenes having a number average molecular weight of 400 to 3000, in another instance of 400 to 2500, and in a further instance of 400 or 450 to 1500. The hydrocarbyl-substituted phenol can be prepared by alkylating a phenol with an olefin or polyolefin described above, such as, a polyisobutylene or polypropylene, using well-known alkylation methods. In some embodiments the phenol may include a lower molecular weight alkyl substituent for example a phenol which carries one or more alkyl chains having a total of less 28 carbon atoms, preferably less than 24 carbon atoms, more preferably less than 20 carbon atoms, preferably less than 18 carbon atoms, preferably less than 16 carbon atoms and most preferably less than 14 carbon atoms. A monoalkyl phenol may be preferred, suitably having from 4 to 20 carbons atoms, preferably 6 to 18, more preferably 8 to 16, especially 10 to 14 carbon atoms, for example a phenol having a C12 alkyl substituent. The aldehyde used to form the Mannich detergent can have 1 to 10 carbon atoms, and is generally formaldehyde or a reactive equivalent thereof such as formalin or paraformaldehyde. The amine used to form the Mannich detergent can be a monoamine or a polyamine. Examples of monoamines include but are not limited to ethylamine, dimethylamine, diethylamine, n-butylamine, dibutylamine, allylamine, isobutylamine, cocoamine, stearylamine, laurylamine, methyllaurylamine, oleylamine, N-methyl-octylamine, dodecylamine, diethanolamine, morpholine, and octadecylamine. Suitable polyamines may be selected from any compound including two or more amine groups. Suitable polyamines include polyalkylene polyamines, for example in which the alkylene component has 1 to 6, preferably 1 to 4, most preferably 2 to 3 carbon atoms. Preferred polyamines are polyethylene polyamines. The polyamine has 2 to 15 nitrogen atoms, preferably 2 to 10 nitrogen atoms, more preferably 2 to 8 nitrogen atoms. In especially preferred embodiments the amine used to form the Mannich detergent comprises a diamine. Suitably it includes a primary or secondary amine which takes part in the Mannich reaction and in addition a tertiary amine. In preferred embodiments component (ii) comprises the product directly obtained from a Mannich reaction and comprising a tertiary amine. For example the amine may comprise a single primary or secondary amine which when reacted in the Mannich reaction forms a tertiary amine which is capable of being quaternised. Alternatively the amine may comprise a primary or secondary amine capable of taking part in the Mannich reaction and also a tertiary amine capable of being quaternised. However component (ii) may comprise a compound which has been obtained from a Mannich reaction and subsequently reacted to form a tertiary amine, for example a Mannich reaction may yield a secondary amine which is then alkylated to a tertiary amine. In some embodiments the compound including a tertiary amino group is (iii) a polyalkylene substituted amine having at least one tertiary amino group. The preparation of compounds including a tertiary amino group of component (iii) is described for example in US2008 / 0113890. The polyalkene-substituted amines having at least one tertiary amino group of the present invention may be derived from an olefin polymer and an amine, for example ammonia, momoamines, polyamines or mixtures thereof. They may be prepared by a variety of methods such as those described and referred to in US2008 / 0113890. Suitable preparation methods are known to the person skilled in the art. The olefin monomers from which the olefin polymers are derived include polymerizable olefin monomers characterised by the presence of one or more ethylenically unsaturated groups for example ethylene, propylene, 1-butene, isobutene, 1-octene, 1,3-butadiene and isoprene. The olefin monomers are usually polymerizable terminal olefins. However, polymerizable internal olefin monomers can also be used to form the polyalkenes. Suitably the polyalkene substituent of the polyalkene-substituted amine is derived from a polyisobutylene. The amines that can be used to make the polyalkene-substituted amine include ammonia, monoamines, polyamines, or mixtures thereof, including mixtures of different monoamines, mixtures of different polyamines, and mixtures of monoamines and polyamines (which include diamines). The amines include aliphatic, aromatic, heterocyclic and carbocylic amines. The monomers and polyamines suitably include at least one primary or secondary amine group. Suitable monoamines are generally substituted with a hydrocarbyl group having 1 to about 50 carbon atoms, preferably 1 to 30 carbon atoms. Saturated aliphatic hydrocarbon radicals are particularly preferred. Examples of suitable monoamines include methylamine, ethylamine, diethylamine, 2-ethylhexylamine, di-(2-ethylhexyl)amine, n-butylamine, di-n-butylamine, allylamine, isobutylamine, cocoamine, stearylamine, laurylamine, methyllaurylamine and oleylamine. Aromatic monoamines include those monoamines wherein a carbon atom of the aromatic ring structure is attached directly to the amine nitrogen. Examples of aromatic monoamines include aniline, di(para-methylphenyl)amine, naphthylamine, and N-(n-butyl)aniline. Examples of aliphatic substituted, cycloaliphatic-substituted, and heterocyclic-substituted aromatic monoamines include: para-dodecylaniline, cyclohexyl-substituted naphthylamine, and thienyl-substituted aniline respectively. Hydroxy amines are also included in the class of useful monoamines. Examples of hydroxylsubstituted monoamines include ethanolamine, di-3-propanolamine, 4-hydroxybutylamine; diethanolamine, and N-methyl-2-hydroxypropylamine. The amine of the polyalkene-substituted amine can be a polyamine. The polyamine may be aliphatic, cycloaliphatic, heterocyclic or aromatic. Examples of suitable polyamines include alkylene polyamines, hydroxy containing polyamines, arylpolyamines, and heterocyclic polyamines. Ethylene polyamines, are especially useful for reasons of cost and effectiveness. Suitable ethylene polyamines are described in relation to the compounds of component (i). Suitable hydroxy containing polyamines include hydroxyalkyl alkylene polyamines having one or more hydroxyalkyl substituents on the nitrogen atoms and can be prepared by reacting alkylenepolyamines with one or more alkylene oxides. Examples of suitable hydroxyalkylsubstituted polyamines include: N-(2-hydroxyethyl)ethylene diamine, N,N-bis(2-hydroxyethyl)ethylene diamine, 1-(2-hydroxyethyl) piperazine, monohydroxypropl-substituted diethylene triamine, dihydroxypropyl-substituted tetraethylene pentamine, propyl and N-(3-hydroxybutyl)tetramethylene diamine. Suitable arylpolyamines are analogous to the aromatic monoamines mentioned above except for the presence within their structure of another amino nitrogen. Some examples of arylpolyamines include N,N’-di-n-butyl-para-phenylene diamine and bis-(para-aminophenyl)methane. Suitable heterocyclic mono- and polyamines will be known to the person skilled in the art. Specific examples of such heterocyclic amines include N-aminopropylmorpholine, N-aminoethylpiperazine, and N,N’-diaminoethylpiperazine. Hydroxy heterocyclic polyamines may also be used for example N-(2-hydroxyethyl)cyclohexylamine, 3-hydroxycyclopentylamine, parahydroxy-aniline and N-hydroxyethylpiperazine. Examples of polyalkene-substituted amines can include: poly(propylene)amine, poly(butene)amine, N,N-dimethylpolyisobutyleneamine; N-polybutenemorpholine, N-poly(butene)ethylenediamine, N-poly(propylene) trimethylenediamine, N-poly(butene)diethylenetriamine, N’,N’-poly(butene)tetraethylenepentamine, and N,N-dimethyl-N’poly(propylene)-1,3 propylenediamine. The number average molecular weight of the polyalkene-substituted amines can range from 500 to 5000, or from 500 to 3000, for example from 1000 to 1500. In some embodiments the compound including a tertiary amino group is (iv) a tertiary amine of formula R5R6R7N, wherein each of R5, R6 and R7 is independently an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group. The tertiary amine compounds of formula R5R6R7N preferably do not include any primary or secondary amine groups. In some embodiments they may be derived from compounds including these groups but preferably these have been subsequently reacted to form additional tertiary amine species. The tertiary amine compound formula R5R6R7N may contain more than one tertiary amine group. However tertiary amine compounds including primary or secondary amine groups are within the scope of the invention provided these groups do not prevent quaternisation of the tertiary amine species. Tertiary amines (iv) for use herein are preferably compounds of formula R5R6R7N, wherein each of R5, R6 and R7 is independently an optionally substituted alkyl, alkenyl, aryl, aralkyl or alkaryl group. R5, R6 and R7 may be the same or different. In some preferred embodiments R5 and R6 are the same and R7 is different. Preferably each of R5 and R6 is independently an optionally substituted alkyl, alkenyl, aryl, aralkyl or alkaryl group having from 1 to 50 carbon atoms, preferably from 1 to 40 carbon atoms, more preferably from 1 to 30 carbon atoms. Each of R5 and R6 may be optionally substituted with one or more groups selected from halo (especially chloro and fluoro), hydroxy, alkoxy, keto, acyl, cyano, mercapto, alkylmercapto, dialkylamino, nitro, nitroso, and sulphoxy. The alkyl groups of these substituents may be further substituted. Preferably each of R5 and R6 is independently an optionally substituted alkyl or alkenyl group. Preferably each of R5 and R6 is independently an optionally substituted alkyl group. In some embodiments each of R5 and R6 is independently an optionally substituted alkyl or alkenyl group having from 1 to 50 carbon atoms, preferably from 1 to 40 carbon atoms, more preferably from 1 to 30 carbon atoms, suitably from 1 to 20 carbon atoms, preferably from 1 to 12 carbon atoms, more preferably from 1 to 10 carbon atoms, suitably from 1 to 8 carbon atoms, for example from 1 to 6 carbon atoms. In some preferred embodiments R5 is an optionally substituted alkyl or alkenyl group, preferably having from 1 to 10, preferably from 1 to 4 carbon atoms. Preferably R5 is an alkyl group. It may be a substituted alkyl group, for example a hydroxy substituted alkyl group. In some preferred embodiments R5 is an unsubstituted alkyl group. The alkyl chain may be straight-chained or branched. Suitably R5 is selected from methyl, ethyl, propyl and butyl, including isomers thereof. Methyl is especially preferred. In some preferred embodiments R5 is a hydroxy substituted alkyl group. Suitably R5 may be selected from hydroxyethyl, hydroxypropyl, hydroxybutyl and isomers thereof. Hydroxyethyl is especially preferred. In some preferred embodiments R6 is an optionally substituted alkyl or alkenyl group, preferably having from 1 to 10, preferably from 1 to 4 carbon atoms. Preferably R6 is an alkyl group. It may be a substituted alkyl group, for example a hydroxy substituted alkyl group. In some preferred embodiments R6 is an unsubstituted alkyl group. The alkyl chain may be straight-chained or branched. Suitably R6 is selected from methyl, ethyl, propyl and butyl, including isomers thereof. Methyl is especially preferred. In some preferred embodiments R6 is a hydroxy substituted alkyl group. Suitably R6 may be selected from hydroxymethyl, hydroxyethyl, hydroxypropyl, hydroxybutyl and isomers hereof. Hydroxyethyl is especially preferred. In some embodiments R7 is an optionally substituted alkyl or alkenyl group having from 1 to 50 carbon atoms, preferably from 1 to 40 carbon atoms, more preferably from 1 to 30 carbon atoms, suitably from 1 to 20 carbon atoms. In some embodiments R7 is an optionally substituted alkyl or alkenyl group having from 1 to 12 carbon atoms, more preferably from 1 to 10 carbon atoms, suitably from 1 to 8 carbon atoms, for example from 1 to 6 carbon atoms. Suitable substituents include halo (especially chloro and fluoro), hydroxy, alkoxy, keto, acyl, cyano, mercapto, alkylmercapto, amino, alkylamino, nitro, nitroso, sulphoxy, amido, alkyamido, imido and alkylimido. The alkyl groups of these substituents may be further substituted. In some embodiments R7 is an optionally substituted alkyl or alkenyl group, preferably having from 1 to 10, preferably from 1 to 4 carbon atoms. Suitably R7 is an optionally substituted alkyl group. In some preferred embodiments R7 is a substituted alkyl group. Preferred substituents include alkoxy and hydroxyl groups. In some preferred embodiments R7 is a hydroxy-substituted alkyl group. The alkyl chain may be straight-chained or branched. In some preferred embodiments R7 is a hydroxyethyl group. In some preferred embodiments R7 is an optionally substituted alkyl or alkenyl group having from 6 to 36 carbon atoms, for example from 8 to 30 carbon atoms, suitably from 10 to 24 carbon atoms, for example 12 to 18 carbon atoms. Suitable tertiary amine compounds of formula R5R6R7N include simple alkylamino and hydroxyalkylamino compounds. Simple alkylamino and hydroxyalkyl amino compounds are preferably compounds of formula r5r6r7N, wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group. Each of R5, R6 and R7 may be the same or different. In some embodiments each of R5, R6 and R7 is independently selected from an alkyl or hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms, for example 1 to 4 carbon atoms. Each of R5, R6 and R7 may be independently selected from methyl, ethyl, propyl, butyl, pentyl, hexyl, hydroxymethyl, hydroxyethyl, hydroxypropyl, hydroxybutyl, hydroxypentyl and hydroxyhexyl. The amine of formula R5R6R7N may be a trialkylamine, a dialkylhydroxyalkylamine, a dihydroxyalkylalkylamine or a trihydroxyalkylamine. There are many different compounds of this type and these will be known to the person skilled in the art. In some embodiments one or more of R5, R6 and R7 may be a polyalkoxy group or a poly alkoxy alkyl group. In such compounds two or more ethylene oxide and / or propylene oxide molecules may react with a primary or secondary amine or with a hydroxy alkyl substituent of a primary, secondary or tertiary amine to form the compound including a tertiary amino group. In some embodiments one or two of the groups R5, R6 and R7 is a short chain alkyl or hydroxy alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or group having 6 to 30, preferably 10 to 24 carbon atoms. In some embodiments each of R5 and R6 is Ci to C4 alkyl, preferably methyl and R7 is an alkyl or alkenyl group having 6 to 36, preferably 10 to 30, for example 12 to 24 carbon atoms. In some embodiments each of R5 and R6 is a Ci to C4 hydroxy alkyl, for example, hydroxy ethyl and R7 is an alkyl or alkenyl group having 6 to 36, preferably 10 to 30, for example 12 to 24 carbon atoms. Compounds of these types include, for example, dimethyl oleylamine, dimethyloctadecylamine, hexadecyl dimethyl amine and N-oleyl diethanolamine. In some embodiments R5 is Ci to C4 alkyl, preferably methyl and each R6 and R7 is an alkyl or or alkenyl group having 6 to 36, preferably 8 to 30, for example 10 to 24 carbon atoms. In some embodiments R5 is Ci to C4 hydroxyalkyl, for example hydroxyethyl, and each R6 and R7 is an alkyl or hydroxy alkyl or alkenyl having 6 to 36, preferably 8 to 30, for example 10 to 24 carbon atoms. Compounds of these types include, for example, N-methyl-N,N-ditallowamine and dicocomethyl amine. Especially preferred tertiary amine compounds of formula R5R6R7N include N,N-dimethyl ethanolamine, dimethyl oleylamine, dimethyloctadecylamine, N-methyl-N,N-ditallowamine, hexadecyl dimethyl amine and N-oleyl diethanolamine. In some embodiments the compound including a tertiary amino group is (v) a cyclic tertiary amine. Suitable cyclic amines are described in the applicant’s earlier application WO2017 / 017454. Other suitable cyclic tertiary amines include the cyclic Mannich reaction products described in US2023 / 0080086. Especially preferred cyclic tertiary amine compounds include methyl pyrrolidine and methyl imidazole. In some embodiments the compound including a tertiary amino group is (vi) a polyetheramine compound. Some preferred polyetheramine compounds are polyoxyalkylene amines. Such compounds may be derived by alkoxylation of an N,N dialkyl hydroxyalkylamine such as N,N dimethyl aminoethanol or N,N dimethylamino propanol. In other embodiments such compounds may be derived by alkoxylation of a C1-C30 alcohol preferably a C4-C20 alcohol followed by amination with ammonia further followed by alkylation of the amine. Such processes are described in US2013225463. Suitable polyetheramine compounds are also described in US2013225463. To form the quaternary ammonium salts used in the present invention the compound including a tertiary amino group (a) is reacted with (b) a quaternising agent either in the presence of (c) itaconic acid or an ester and / or polymer thereof; or followed by an anion exchange reaction with (c) itaconic acid or an ester and / or polymer thereof. In some embodiments component (b) comprises an epoxide. Any suitable epoxide compound may be used. Suitable epoxide compounds are those of formula (14): O (14) wherein each of R8, R9, R10, R11 is independently selected from hydrogen or an optionally substituted alkyl, alkenyl or aryl group, provided at least one of R8, R9, R10 and R11 is hydrogen. Preferably at least two R8, R9, R10 and R11 are hydrogen. Most preferably three of R8, R9, R10 and R11 are hydrogen or R8, R9, R10 and R11 may be all hydrogen. In the structure above and the definitions which follow R8 and R9 are interchangeable and thus when these groups are different either enantiomer or diastereomer may be used. In the structure above and the definitions which follow R10 and R11 are interchangeable and thus when these groups are different either enantiomer or diastereomer may be used. Preferably R8 is hydrogen or an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group. R8 may suitably be selected from hydrogen and phenyl. Most preferably R8 is hydrogen. Preferably R9 is hydrogen or an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group. Most preferably R9 is hydrogen. Preferably R10 is hydrogen or an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group. Most preferably R10 is hydrogen. Preferably R11 is hydrogen or an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group. In some preferred embodiments R11 is an optionally substituted aryl group. For example R11 may be phenyl. In some preferred embodiments R11 is an optionally substituted alkyl or alkenyl group. R11 may be an alkyl group, for example an unsubstituted alkyl group. R11 may be an alkyl group having 1 to 50 carbon atoms, preferably from 1 to 30 carbon atoms, suitably 1 to 20 carbon atoms, preferably from 1 to 12 carbon atoms, for example from 1 to 8 or from 1 to 4 carbon atoms. In some embodiments R11 is hydrogen. In some embodiments R11 is the moiety CH2OR12 or CH2OCOR13 wherein each of R12 and R13 may be an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group. R12 is preferably an optionally substituted alkyl or aryl group, preferably having from 1 to 30 carbon atoms, preferably from 1 to 20 carbon atoms, suitably from 1 to 12 carbon atoms. When R12 is an alkyl group it may be straight-chained or branched. In some embodiments it is branched. R12 may be an optionally substituted phenyl group. In one embodiment R12 is a 2-methyl phenyl group. In another embodiment R12 is CH2C(CH2CH3)CH2CH2CH2CH3. R13 may be an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group. R13 is preferably an optionally substituted alkyl or aryl group, preferably having from 1 to 30 carbon atoms, preferably from 1 to 20 carbon atoms, suitably from 1 to 12 carbon atoms. When R13 is an alkyl group it may be straight-chained or branched. In some preferred embodiments it is branched. R13 may be an optionally substituted phenyl group. In one embodiment R13 is C(CH3)R2 wherein each R is an alkyl group. The R groups may be the same or different. Preferably R13 is an alkyl group having 1 to 5 carbon atoms. In some embodiments R13 may include an oxygen atom in the carbon chain, i.e. R13 may include an ether functional group. Suitable epoxide compounds include ethylene oxide, propylene oxide, butylene oxide, pentylene oxide, hexylene oxide, heptylene oxide, dodecylene oxide, alkyl glycidyl ethers, for example 2-ethylhexyl glycidyl ether or isopropyl glycidyl ether, alkyl glycidyl esters, styrene oxide, stilbene oxide and other C2 to C30 hydrocarbyl groups. Some preferred epoxide compounds include styrene oxide, ethylene oxide, propylene oxide, butylene oxide, stilbene oxide, dodecylene oxide 2-ethylhexyl glycidyl ether and isopropyl glycidyl ether. Styrene oxide, butylene oxide, 2-ethylhexyl glycidyl ether and propylene oxide are especially preferred. In some embodiments when component (b) is an epoxide this may be reacted in combination with an acid which is not component (c), provided that this is followed by a subsequent anion exchange reaction with itaconic acid or an ester and / or polymer thereof. In such embodiments any suitable acid may be used and these will be known to the person skilled in the art. One preferred such acid is acetic acid. However, in preferred embodiments when component (b) comprises an epoxide this is reacted with component (a) in the presence of a component (c). In some embodiments component (b) is not an epoxide. In such embodiments the quaternising agent is reacted with component (a) before a subsequent anion exchange reaction with component (c). In such embodiments the quaternising agent may suitably be selected from an ester of a carboxylic acid, dialkyl sulfates, benzyl halides, hydrocarbyl substituted carbonates, alkyl halides or mixtures thereof. In preferred such embodiments component (b) is selected from a carbonate compound of formula R14COOR15 wherein each R14 and R15 is an optionally substituted hydrocarbyl group or an ester of a carboxylic acid. Each of R14 and R15 is preferably an optionally substituted alkyl alkenyl or aryl group having up to 30 carbon atoms. Preferably each of R14 and R15 is an optionally substituted alkyl group. Preferably each of R14 and R15 is an alkyl group having up to 24 carbon atoms, preferably up to 20 carbon atoms, suitably up to 16 carbon atoms, preferably up to 12 carbon atoms, suitably up to 8, for example up to 6 or up to 4 carbon atoms. Preferably each of R14 and R15 is an unsubstituted alkyl group. R14 may be the same or different to R15. Preferably R14 is the same as R15 Preferred carbonates are dimethyl carbonate and diethyl carbonate. Dimethyl carbonate is especially preferred. In some embodiments component (b) is an ester of a carboxylic acid. Preferred ester quaternising agents are compounds of formula (15): O (15) in which R16 is an optionally substituted alkyl, alkenyl, aryl or alkylaryl group and R17 is a C1 to C22 alkyl, aryl or alkylaryl group. The compound of formula (15) is suitably an ester of a carboxylic acid capable of reacting with a tertiary amine to form a quaternary ammonium salt. Suitable quaternising agents include esters of carboxylic acids having a pKa of 3.5 or less. The compound of formula (15) is preferably an ester of a carboxylic acid selected from a substituted aromatic carboxylic acid, an a-hydroxycarboxylic acid and a polycarboxylic acid. In some preferred embodiments the compound of formula (15) is an ester of a substituted aromatic carboxylic acid and thus R16 is a substituted aryl group. Preferably R16 is a mono-substituted aryl group. Preferably R16 is an ortho substituted aryl group. Suitably R16 is substituted with a group selected from OH, NH2, NO2 or COOMe. Preferably R16 is substituted with an OH or NH2 group. Suitably R16 is a hydroxy substituted aryl group. Most preferably R16 is a 2-hydroxyphenyl group. Preferably R17 is an alkyl, aralkyl or alkaryl group. R17 may be a C1 to C16 alkyl group, preferably a C1 to C10 alkyl group, suitably a C1 to C8 alkyl group. R17 may be C7 to C16 aralkyl or alkaryl group, preferably a C7 to C10 aralkyl or alkaryl group. R17 may be methyl, ethyl, propyl, butyl, pentyl, benzyl or an isomer thereof. Preferably R17 is benzyl or methyl. Most preferably R17 is methyl. Especially preferred compounds of formula (15) are lower alkyl esters of salicylic acid such as methyl salicylate, ethyl salicylate, n and I propyl salicylate, and butyl salicylate, preferably methyl salicylate. In some embodiments the compound of formula (15) is an ester of an a-hydroxycarboxylic acid. In such embodiments the compound has the structure (16): OH R18-C--COOR17 I R19 (16) wherein R17 is as defined above and R18 and R19 are the same or different and each is selected from hydrogen, alkyl, alkenyl, aralkyl or aryl. Compounds of this type suitable for use herein are described in EP 1254889. In some embodiments the compound of formula (15) is an ester of a polycarboxylic acid. In this definition we mean to include dicarboxylic acids and carboxylic acids having more than 2 acidic moieties. In such embodiments R16COO is preferably present in the form of an ester, that is the one or more further acid groups present in the group R17 are in esterified form. However embodiments in which not all acid groups are esterified are also encompassed. Mixed esters of polycarboxylic acids may also be used. Preferred esters are C1 to C4 alkyl esters. The ester quaternising agent may be selected from the diester of oxalic acid, the diester of phthalic acid, the diester of maleic acid, the diester of malonic acid or the diester of citric acid. One especially preferred compound of formula (15) is dimethyl oxalate. In preferred embodiments the compound of formula (15) is an ester of a carboxylic acid having a pKa of less than 3.5. In such embodiments in which the compound includes more than one acid group, we mean to refer to the first dissociation constant. The ester quaternising agent may be selected from an ester of a carboxylic acid selected from one or more of oxalic acid, phthalic acid, salicylic acid, maleic acid, malonic acid, citric acid, nitrobenzoic acid, aminobenzoic acid and 2, 4, 6-trihydroxybenzoic acid. Preferred ester quaternising agents include dimethyl oxalate, methyl 2-nitrobenzoate and methyl salicylate. Dimethyl oxalate and methyl salicylate are especially preferred. Component (c) provides the itaconic acid based anion An-. By an itaconic acid based anion we mean to refer to an anion which is obtained from itaconic acid or a related compound. Such related compounds include anhydrides and esters of itaconic acid and polymers of itaconic acid, itaconic anhydride or itaconate esters. Preferably the polymers include repeat units derived only from itaconic acid, itaconic anhydride or itaconate esters, i.e. they do not include units derived from a different monomer type. Thus, the polymers preferably consist essentially of or consist of repeat units derived from itaconic acid, itaconic anhydride or itaconate esters. Component (c) comprises itaconic acid or an ester and / or polymer thereof. Thus component (c) may comprise itaconic acid; component (c) may comprise an ester of itaconic acid; component (c) may comprise a polymer of itaconic acid or itaconic anhydride: or component (c) may comprise a polymeric ester of itaconic acid. Also within the scope of the invention are embodiments in which the anion An- is an oligomeric species obtained from, for example, 2, 3 or 4 itaconic acid moieties. For the avoidance of doubt component (c) may comprise one or more than one reaction product and / or one or more than one polymer. References herein to “the” or “a” reaction product and / or “the” or “a” polymer include embodiments in which mixtures of two or more reaction products and / or polymers are present. Itaconic acid has the formula (17): (17) Itaconic anhydride has the formula (18): (18) In some embodiments component (c) may comprise itaconic acid. In some embodiments component (c) may comprise an ester of itaconic acid. The skilled person will appreciate that to form an anion An- component (c) must include at least one residual carboxylic acid functional group. When component (c) comprises a monomeric ester of itaconic acid it suitably comprises a mixture of compounds of formula (19) and (20): wherein R20 is an optionally substituted hydrocarbyl group. Such esters may be prepared by reaction of itaconic acid or itaconic anhydride with an alcohol R20OH. In preferred embodiments component (c) is a polymeric species. In some embodiments component (c) comprises a polymer of itaconic acid. In some embodiments component (c) comprises a polymer of itaconic anhydride which is at least partially hydrolysed. In some embodiments component (c) comprises a polymer prepared from an alcohol and itaconic acid or itaconic anhydride. Such a polymer may be prepared by polymerising itaconic acid or itaconic anhydride and then optionally esterifying the polymerised itaconic acid or anhydride. When an anhydride is used the polymer may optionally be hydrolysed to provide acid residues prior to reaction with the alcohol. Suitable hydrolysis conditions will be known to the person skilled in the art. In preferred embodiments the polymer is prepared by polymerising the reaction product of itaconic acid or itaconic anhydride and an alcohol. Thus in this embodiment the polymer is preferably prepared by forming an ester of itaconic acid and an alcohol; and then polymerising the ester. The ester may be a mixture of esters, such as a mixture of compounds of formula (19) and (20). Polymerisation is preferably carried out by a free radical initiated process. Component (c) preferably comprises a polymer of formula (21): wherein z is at least 2; x + y = 1; and each R is independently hydrogen or an optionally substituted hydrocarbyl group provided that, in the polymer as a whole, not all of the R groups are a hydrocarbyl group. In the compound of formula (21) in each repeat unit one, none or both groups R may be a hydrocarbyl group. However in the molecule as a whole at least one group R is not a hydrocarbyl group i.e. the polymer of formula (21) includes at least one free acid group. In the polymer of formula (21) z is at least 2. Preferably z is at least 4, preferably at least 6, more preferably at least 8, for example at least 10. Suitably z is from 10 to 200, preferably from 15 to 80, more preferably from 20 to 60, for example from 25 to 50. x + y = 1. Thus in each repeat unit one of x and y is 0 and the other is 1. Component (c) preferably comprises the polymerised reaction product of itaconic acid / anhydride and an alcohol. Any suitable alcohol may be used to prepare component (c). Preferred alcohols have at least 4 carbon atoms. The alcohol may be a monohydric alcohol or a polyhydric alcohol. Monohydric alcohols are preferred. Suitably the alcohol may be a compound of formula H-(OR22)m-OR21, wherein R22 is an optionally substituted alkylene or arylene group; R21 is hydrogen or an optionally substituted hydrocarbyl group; and m is 0 or a positive integer; provided that m is not 0 when R21 is hydrogen. In the compound of formula (21), each group R is suitably hydrogen or a group of formula (OR22)mOR21. In some embodiments m is 0 and component (c) may be formed from an alcohol of formula R21OH. In such embodiments R21 is an optionally substituted hydrocarbyl group. Preferably R21 is an optionally substituted alkyl, alkenyl, aryl, aralkyl or alkaryl group. R21 is preferably an optionally substituted hydrocarbyl group having at least 4 carbon atoms. Preferably R21 is an optionally substituted hydrocarbyl group having 5 to 200 carbon atoms, suitably 6 to 50 carbon atoms, preferably 8 to 30 carbon atoms. Most preferably R21 is an optionally substituted hydrocarbyl group having 4 to 10 carbon atoms. R21 may be an optionally substituted alkyl, alkenyl or aryl group having at least 5 carbon atoms. In some embodiments R21 is an optionally substituted C5 to C200 alkyl or alkenyl group, preferably a Ce to C50 alkyl or alkenyl group, preferably a Ca to C30 alkyl or alkenyl group. R21 may be substituted with one or more groups selected from halo (e.g. chloro, fluoro or bromo), nitro, hydroxy, mercapto, sulfoxy, amino, nitryl, acyl, carboxy, alkyl (e.g. Ci to C4 alkyl), alkoxyl (e.g. Ci to C4 alkoxy), amido, keto, sulfoxy and cyano. In some embodiments R21 has at least 6 carbon atoms. In some embodiments R21 has at least 8 carbon atoms. R21 may have more than 8 carbon atoms. In some embodiments R21 may have more than 10 carbon atoms, for example more than 12 carbon atoms, more than 14 carbon atoms or more than 16 carbon atoms. In some embodiments R21 has less than 30 carbon atoms, preferably less than 28 carbon atoms, suitably less than 26 carbon atoms. In some embodiments R21 has less than 12 carbon atoms. In some preferred embodiments R21 is an alkyl or alkenyl group having 6 to 50 carbon atoms, preferably 8 to 30 carbon atoms. Preferably R21 is an unsubstituted alkyl or alkenyl group. In some preferred embodiments R21 is an unsubstituted alkenyl group. R21 may be straight chained or branched. In some embodiments R21 is an unsubstituted straight chained or branched alkyl or alkenyl group, having 4 to 50 carbon atoms, preferably 6 to 30 carbon atoms. In some embodiments R21 is an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group having less than 20 carbon atoms, suitably less than 16 carbon atoms. In some embodiments R21 is an alkyl, alkenyl, aryl, alkaryl or aralkyl group having 4 to 10 carbon atoms. In some embodiments R21 is an alkyl, alkenyl, aryl, alkaryl or aralkyl group having 6 to 16 carbon atoms. In some embodiments R21 is an unsubstituted alkyl, aryl, alkaryl or aralkyl group having less than 16 carbon atoms. In some embodiments R21 is an unsubstituted alkyl, aryl, alkaryl or aralkyl group having less than 12 carbons, suitably less than 10 carbon atoms. In some embodiments R21 is an alkaryl group. In one embodiment R21 is benzyl. In some embodiments R21 is an alkyl group, preferably an unsubstituted alkyl group having 6 to 50, preferably 8 to 30 carbon atoms, for example 12 to 24 carbon atoms. In some embodiments R21 is a group CH3(CH2)x wherein x is from 4 to 23, preferably from 9 to 19. In some preferred embodiments, R21 is a C12 to C18 alkyl group. In some preferred embodiments, R21 is a C4 to C10 alkyl group. R21 may be a straight chain, branched or cyclic alkyl group. Suitable alcohols R21OH for use herein include hexanol, octanol, nonanol, decanol, dodecanol, tetradecanol, cetyl alcohol, stearyl alcohol, 2-ethyl-1-butanol, 2-ethyl-1-hexanol, 2-ethyl-1-heptanol, 2-propylheptanol, 2-ethyl-1-decanol, 2-hexyl-1-decanol, 2-octyl-1-decanol, 2-hexyl-1- dodecanol, 2-octyl-1-dodecanol, 2-decyl-1-tetradecanol, isotridecanol, cyclohexanol, cyclooctanol and benzyl alcohol. In some embodiments R21 is an alkenyl group, preferably an unsubstituted alkenyl group having 5 to 36 carbon atoms, more preferably 10 to 30 carbon atoms, suitably 10 to 24 carbon atoms. R21 may be a straight chain, branched or cyclic alkenyl group. Suitable alkenyl alcohols include citronellol, oleyl alcohol, 9-decen-1-ol, cis-3-hexen-1-ol, trans-2-hexen-1-ol, 5-hexen-1-ol, 6-methyl-5-hepten-2-ol, 1-octen-3-ol, trans-2-octen-1-ol and 10-undecen-1-ol. In some embodiments, the alkenyl alcohol is obtainable from a naturally occurring fatty acid, for example by chemical reduction. Such materials may comprise mixtures of alkenyl alcohols. Examples include oleyl alcohol, linoleyl alcohol, and fatty alcohols derived from fatty acids, for example tall oil, coconut oil or palm kernel oil fatty acids. In some embodiments, the alkenyl alcohol may be derived from terpenes. Examples of such alkenyl alcohols include linalool, fenchyl alcohol, terpineol, borneol, isoborneol, citrol, geraniol, citronellol, phytol and nerol. In some preferred embodiments, the alcohol is a Cis alcohol, for example stearyl alcohol or oleyl alcohol. In some embodiments oleyl alcohol is especially preferred. In some preferred embodiments, R21 is a branched, saturated alkyl group, such as a branched, saturated C5 to C24 alkyl group. Suitable branched alcohols for use herein include 2-ethyl-1-butanol, 2-ethyl-1-hexanol, 2-ethyl-1-heptanol, 2-propylheptanol, 2-ethyl-1-decanol, 2-hexyl-1-decanol, 2-octyl-1-decanol, 2-hexyl-1-dodecanol, 2-octyl-1-dodecanol, 2-decyl-1-tetradecanol and isotridecanol. In some embodiments 2-ethyl hexanol is especially preferred. The skilled person will appreciate that commercial sources of alcohols of formula R21OH will often contain mixtures of compounds, for example mixtures of isomers and / or mixtures of homologues. Some suitable alcohols for use herein include mixed Cie to Cis monounsaturated alcohols, known as cetostearyl alcohol. In some embodiments m is not 0 and component (c) may suitably be formed from an alcohol of formula H-(OR22)m-OR21. In such embodiments R21 is hydrogen an optionally substituted hydrocarbyl group. R21 may be as defined above. R22 is an optionally substituted arylene or alkylene group. Preferably R2 is an optionally substituted alkylene group. Preferably R22 is an unsubstituted alkylene group. Preferably R22 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 R22 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 R22 may be an ethylene, propylene, butylene, pentylene, or hexylene group. When R2 has more than 2 carbon atoms any isomer may be present. Preferably R2 is an ethylene or a propylene group, most preferably a propylene group. R22 may comprise a mixture of isomers. For example when R22 is propylene, the polyhydric alcohol may include moieties -CH2CH(CH3)- and -CH(CH3)CH2- in any order within the chain. R22 may comprise a mixture of different groups for example ethylene, propylene or butylene units. Block copolymer units are preferred in such embodiments. R22 is preferably an ethylene, propylene or butylene group. R22 may be an n-propylene or n-butylene group or an isopropylene or isobutylene group. For example R22 may be -CH2CH2-, -CH2CH(CH3)-, -CH2C(CH3)2, -CH(CH3)CH(CH3)- or-CH2CH(CH2CH3)-. Preferably R22 is ethylene or propylene. More preferably R22 is -CH2CH2- or -CH(CH3)CH2-. Most preferably R22 is -CH(CH3)CH2-. In some embodiments m is at least 1. Preferably n is from 1 to 200, preferably from 1 to 50, more preferably from 1 to 30, more preferably from 1 to 24, preferably from 1 to 20, suitably from 1 to 16. In some preferred embodiments m is from 8 to 20. The skilled person will appreciate that commercial sources of alcohols of formula H-(OR22)m-OR21 often contain mixtures of compounds, for example in which m may be between 10 and 20. In preferred embodiments in which m is not 0, R21 is an optionally substituted alkyl, alkenyl or aryl group, suitably an optionally substituted alkyl or alkenyl group. Preferably R21 has from 4 to 50 carbon atoms, preferably 4 to 40 carbon atoms, more preferably from 10 to 30 carbon atoms. R21 may be straight chain or branched. Preferably R21 is straight chain. In some embodiments R21 is a substituted alkyl or alkenyl group, suitably a substituted alkyl group. Suitable substituents are hydroxy and ester groups. In some embodiments R21 is a 2-hydroxy alkyl, alkenyl or aryl group. Suitably R21 is an unsubstituted alkyl or alkenyl group. Preferably R21 is an alkyl group, preferably an unsubstituted alkyl group. Suitably R21 is selected from an alkyl group having from 1 to 40, preferably 6 to 30, more preferably 10 to 20 carbon atoms. In some embodiments R21 is a C4 to C30 alkyl or alkenyl group, m is not 0 and component (c) is prepared from an alkyl or alkenyl ether of a polyhydric alcohol, for example an ether of a polyethylene glycol, a polypropylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol or tripropylene glycol. Some especially preferred alcohols for use in preparing component (c) are of the formula CH3(CH2)aO(CH2CH(CH3)O)bH or an isomer thereof wherein a is from 4 to 30, preferably from 8 to 20, more preferably from 10 to 15, and b is from 1 to 30, preferably from 5 to 25, more preferably from 10 to 20. In one preferred embodiment a is 13 and b is 15. The alcohol of formula H-(OR22)m-OR21 may be selected from: - alkanols of formula CH3(CH2)aOH or an isomer thereof wherein a is from 4 to 23; - branched or cyclic alkyl alcohols in which m is 0 and R21 has 6 to 24 carbon atoms; - alkenyl alcohols in which n is 0 and R21 has 6 to 24 carbon atoms; - glycol ethers in which m is not 0. Preferred alkanols of formula CH3(CH2)aOH include stearyl alcohol, tetradecanol, cetyl alcohol, octanol, hexanol, nonanol, decanol and dodecanol. Preferred branched or cyclic alkyl alcohols in which m is 0 include cyclohexanol, cyclooctanol, 2-propylheptanol, 2-ethyl-1-hexanol, 2-ethyl-1-heptanol, 2-propylheptanol, 2-ethyl-1-decanol, 2-ethyl-1-butanol, 2-hexyl-1-decanol, 2-octyl-1-decanol, 2-hexyl-1-dodecanol, 2-octyl-1-dodecanol, 2-decyl-1-tetradecanol and isotridecanol. Preferred alkenyl alcohols in which m is 0 include citronellol, oleyl alcohol, 9-decen-1-ol, cis-3-hexen-1-ol, trans-2-hexen-1-ol, 5-hexen-1-ol, 6-methyl-5-hepten-2-ol, 1-octen-3-ol, trans-2-octen-1-ol and 10-undecen-1-ol. Preferred glycol ethers in which m is not 0 include compounds of formula CH3(CH2)aO(CH2CH(CH3)O)bH or an isomer thereof wherein a is from 10 to 15, and b is from 10 to 20. Preferably component (c) is prepared by reacting itaconic acid or itaconic anhydride and an alcohol and then optionally polymerising the resultant ester. The optional polymerisation may be carried out in situ or in a subsequent reaction step. The itaconic acid / anhydride and alcohol are preferably reacted in a molar ratio of from 15:1 to 1:15, suitably from 10:1 to 1:10, preferably from 5:1 to 1:5, more preferably from 2:1 to 1:2, for example from 1.5:1 to 1:1.5 or from 1.2:1 to 1:1.2. For the avoidance of doubt, reference to molar ratios are to the number of moles of each molecule reacted, not the number of functional groups reacted. Thus a 1:1 molar ratio refers to one mole of itaconic acid / anhydride reacting with one mole of alcohol. Most preferably itaconic acid / anhydride and the alcohol are reacted in an approximately 1:1 molar ratio. By “approximately” unless otherwise stated herein we mean within 10% of the values specified. The itaconic acid / anhydride and the alcohol react to form an ester. In preferred embodiments the itaconic acid / anhydride and the alcohol are reacted in an approximately 1:1 molar ratio. The reaction product of the itaconic acid / anhydride and the alcohol may comprise a mixture of compounds. Preferably the reaction product comprises predominantly monoesters. However some diester may also be present, along with unreacted itaconic acid / anhydride. As the skilled person will appreciate as illustrated in figures (19) and (20) above two different monoesters can be formed even when a single alcohol is used. Mixtures of alcohols can also be used leading to further mixtures in the product. In some embodiments in which the alcohol is a diol the reaction product may comprise oligomers. Suitable conditions for carrying out the esterification reaction will be known to those skilled in the art. In some preferred embodiments an acid catalyst is used. In some embodiments the reaction product obtained following reaction of itaconic acid / anhydride and an alcohol is used directly as component (c). In some embodiments the reaction product obtained following reaction of the itaconic acid / anhydride and an alcohol is then polymerised. In some embodiments component (c) may be prepared by polymerising itaconic acid / anhydride and then esterifying some of the acid groups on the polymeric acid. Component (c) preferably comprises a polymerised ester of formula (21). In the polymeric compounds defined in formula (21), preferably at least 10% of all R groups are hydrogen. Thus at least 10% of all acid residues in the molecule are in the form of the free carboxylic acid COOH. For the avoidance of doubt, references to the number of groups which are hydrogen is a molar ratio rather than a weight ratio. Preferably least 15% of all R groups in the compound of formula (21) are hydrogen, more preferably at least 20%, suitably at least 25%, more preferably at least 30%, for example at least 35% or at least 40% of all R groups in the compound of formula (21) are hydrogen. Up to 100% of all R groups may be hydrogen, for example up to 95%, suitably up to 90%, preferably up to 80%, more preferably up to 75%, for example up to 70%, up to 65% or up to 60% of all R groups in the compound of formula (21) are hydrogen. Preferably from 30 to 70% of all R groups in the compound of formula (21) are hydrogen, preferably from 40 to 60%, more preferably from 45 to 55%. In preferred embodiments approximately half of all R groups in the compound of formula (21) are hydrogen. Thus approximately half of the acid groups present in the compound of formula (21) are esterified. R groups that are not hydrogen are an optionally substituted hydrocarbyl group as previously defined herein. Suitably each R group in the compound of formula (21) and the compounds of formula (19) and (20) are is the residue of an alcohol of formula H-(OR22)m-OR21 wherein R21 and R22 are as previously defined. In preferred embodiments in which the compound of formula (21) is prepared by polymerising the reaction product of itaconic acid / anhydride and an alcohol, the compound is prepared by polymerising predominantly monoesters. In the structure shown in formula (21), in each monomer unit preferably one R group is hydrogen and the other is an optionally substituted hydrocarbyl group. Polymerisation of itaconic acid / anhydride or an itaconate is suitably achieved by the addition of a radical initiator. Suitably radical initiators will be known to those skilled in the art and include: azo compounds, for example azobisisobutyronitrile (AIBN); hydroperoxides, for example cumene hydroperoxides, tertiary butyl hydroperoxide, methyl ethyl ketone hydroperoxides; peroxides, for example di-tertiary butyl peroxide, tert-Butyl peroxy pivalate, di cumyl peroxide, benzoyl peroxide 1,1' azobis(cyclohexanecarbonitrile) (ABCN); and persulfates, for example ammonium persulfate, sodium persulfate or potassium persulfate. Suitable amounts of radical initiator and reaction conditions will be known to the person skilled in the art. Component (c) is preferably the optionally polymerised reaction product of itaconic acid / anhydride and an alcohol. In some embodiments component (c) is the reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted hydrocarbyl group having 4 to 30, preferably 6 to 24, carbon atoms. In some embodiments component (c) is the reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is a (preferably branched) alkyl group having 4 to 30, preferably 6 to 24, carbon atoms. In some embodiments component (c) is the reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 4 to 30, preferably 6 to 24, carbon atoms. In some embodiments component (c) is the reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an alkenyl group having 4 to 30, preferably 6 to 24, carbon atoms. In some embodiments component (c) is the reaction product of itaconic acid / anhydride and an alcohol of formula H-(OR22)m-OR21 wherein m is from 1 to 24, R is ethylene, propylene or isopropylene, and R21 is an unsubstituted alkyl group having 4 to 30, preferably 6 to 24, carbon atoms. In some embodiments component (c) is the reaction product of itaconic acid / anhydride and an alcohol selected from 2-ethyl-1-butanol, 2-ethyl-1-hexanol, 2-ethyl-1-heptanol, 2-propylheptanol, 2-ethyl-1-decanol, 2-hexyl-1-decanol, 2-octyl-1-decanol, 2-hexyl-1-dodecanol, 2-octyl-1-dodecanol, 2-decyl-1-tetradecanol and isotridecanol. In some embodiments component (c) is the reaction product of itaconic acid / anhydride and an alcohol of formula H-(OR22)m-OR21 wherein m is from 1 to 24, R is ethylene, propylene or isopropylene, and R21 is an unsubstituted alkyl group having 4 to 30, preferably 6 to 24, carbon atoms. In some embodiments component (c) is the reaction product of itaconic acid / anhydride and an alkenyl alcohol selected from citronellol, oleyl alcohol, 9-decen-1-ol, cis-3-hexen-1-ol, trans-2-hexen-1-ol, 5-hexen-1-ol, 6-methyl-5-hepten-2-ol, 1-octen-3-ol, trans-2-octen-1-ol and 10-undecen-1-ol. In some embodiments component (c) is the reaction product of itaconic acid / anhydride and citronellol or oleyl alcohol (preferably oleyl alcohol). In some especially preferred embodiments component (c) is the reaction product of itaconic acid / anhydride and 2-ethylhexanol. In some embodiments component (c) is the polymerised reaction product of a dicarboxylic acid compound of formula (I) or an anhydride thereof; and an alcohol of formula R21OH wherein R21 is an optionally substituted hydrocarbyl group having 4 to 30, preferably 6 to 24, carbon atoms. In some embodiments component (c) is the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is a (preferably branched) alkyl group having 4 to 30, preferably 6 to 24, carbon atoms. In some embodiments component (c) is the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is a (preferably branched) alkyl group having 4 to 10 carbon atoms. In some embodiments component (c) is the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 6 to 30, preferably 4 to 24, carbon atoms. In some embodiments component (c) is the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an alkenyl group having 4 to 30, preferably 6 to 24, carbon atoms. In some embodiments component (c) is the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula H-(OR22)m-OR21 wherein m is from 1 to 24, R22 is ethylene, propylene or isopropylene, and R21 is an unsubstituted alkyl group having 4 to 30, preferably 6 to 24, carbon atoms. In some embodiments component (c) is the polymerised reaction product of itaconic acid / anhydride and an alcohol selected from 2-ethyl-1-butanol, 2-ethyl-1-hexanol, 2-ethyl-1-heptanol, 2-propylheptanol, 2-ethyl-1-decanol, 2-hexyl-1-decanol, 2-octyl-1-decanol, 2-hexyl-1-dodecanol, 2-octyl-1-dodecanol, 2-decyl-1-tetradecanol and isotridecanol. In some embodiments component (c) is the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula H-(OR22)m-OR21 wherein m is from 1 to 24, R22 is ethylene, propylene or isopropylene, and R21 is an unsubstituted alkyl group having 4 to 30, preferably 6 to 24, carbon atoms. In some embodiments component (c) is the polymerised reaction product of itaconic acid / anhydride and an alkenyl alcohol selected from citronellol, oleyl alcohol, 9-decen-1-ol, cis-3-hexen-1-ol, trans-2-hexen-1-ol, 5-hexen-1-ol, 6-methyl-5-hepten-2-ol, 1-octen-3-ol, trans-2-octen-1-ol and 10-undecen-1-ol. In some embodiments component (c) is the polymerised reaction product of itaconic acid / anhydride and citronellol or oleyl alcohol (preferably oleyl alcohol). In some especially preferred embodiments component (c) is the polymerised reaction product of itaconic acid / anhydride and 2-ethylhexanol. In some preferred embodiments component (c) is the polymerised reaction product of itaconic acid / anhydride and 2-ethylhexanol wherein the polymer has a weight average molecular weight of from 2000 to 50000, preferably from 4000 to 30000, more preferably from 5000 to 20000, for example from 6000 to 15000, suitably from 8000 to 12000. Weight average molecular weight may be measured by gel permeation chromatography. The quaternary ammonium salt of the invention is suitably prepared by reacting: (a) a compound including a tertiary amino group; (b) a quaternising agent; and (c) itaconic acid or an ester and / or polymer thereof; wherein component (c) is either added concurrently with the quaternising agent or in a subsequent separate step. Components (a) and (b) are suitably reacted in a molar ratio of from 10:1 to 1:10; preferably from 5:1 to 1:5; more preferably from 2:1 to 1:2; for example from 1.5:1 to 1:1.5. Preferably components (a) and (b) are reacted in an approximately 1:1 molar ratio, based on the number of tertiary amino groups in component (a). In some embodiments components (a) and (b) are suitably reacted in the presence of component (c). In some embodiments component (c) is used in a subsequent step, after components (a) and (b) have been reacted. Component (c) may be a polymeric species. Component (c) is suitably reacted in a molar ratio based on the number of acid groups COOH present, compared with the number of tertiary amino group present in component (a). In most embodiments molecules of component (a) comprises only a single tertiary amino group. However embodiments in which multiple tertiary amino groups are within one molecule are within the scope of the invention. Preferably component (c) is reacted in a molar ratio to component (a) of from 10:1 to 1:10; preferably from 5:1 to 1:5; more preferably from 3:1 to 1:3 based on the number of free acid groups in component (c) and tertiary amino groups in component (a). In some embodiments component (c) is reacted in a molar ratio to component (a) of from 1.5:1 to 2.5:1; suitably about 2:1 based on the number of free acid groups in component (c) and tertiary amino groups in component (a). In such embodiments approximately half of the acid groups present in the anion would be neutralised. In some embodiments component (c) is reacted in a molar ratio to component (a) of from 1.5:1 to 1:1.5; suitably about 1:1 based on the number of free acid groups in component (c) and tertiary amino groups in component (a). In such embodiments substantially of the acid groups present in the anion would be neutralised. In preferred embodiments the quaternary ammonium salt of the present invention comprises polyvalent anions including multiple carboxylate anions and a plurality of quaternary ammonium cations. Such a salt may be represented by the formula (Q+)nAn-. Embodiments in which multiple cationic species Q+ are derived from the same molecule are also within the scope of the invention. In preferred embodiments the or each quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group; and (b) a quaternising agent selected from epoxide; esters of a carboxylic acid and hydrocarbyl carbonate compounds; and comprises anions An- derived from (c) a polymeric ester of itaconic acid. In preferred embodiments the or each quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group which is the reaction product of a hydrocarbyl-substituted acylating agent and a compound having at least one tertiary amine group and a primary amine, secondary amine or alcohol group; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) a polymeric ester of itaconic acid. In some preferred embodiments the or each quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N, wherein each of R5, R6 and R7 is independently an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) a polymeric ester of itaconic acid. In some preferred embodiments the or each quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 6 to 30, preferably 6 to 24, carbon atoms. In some preferred embodiments the or each quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N, wherein each of R5, R6 and R7 is independently an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 6 to 30, preferably 6 to 24, carbon atoms. In some preferred embodiments the or each quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 4 to 10 carbon atoms. In some preferred embodiments the or each quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N, wherein each of R5, R6 and R7 is independently an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 4 to 10 carbon atoms. In some preferred embodiments the quaternary ammonium salt comprises cations Q+ which are is the reaction product of: (a) a compound including a tertiary amino group; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and 2-ethylhexanol. In some preferred embodiments quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N, wherein each of R5, R6 and R7 is independently an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and 2-ethylhexanol. In some preferred embodiments quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or group having 6 to 30, preferably 10 to 24 carbon atoms; and (b) a quarternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) a polymeric ester of itaconic acid. In some preferred embodiments quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or group having 6 to 30, preferably 10 to 24 carbon atoms; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 6 to 30, preferably 6 to 24, carbon atoms. In some preferred embodiments quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or group having 6 to 30, preferably 10 to 24 carbon atoms; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 4 to 10 carbon atoms. In some preferred embodiments the quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or group having 6 to 30, preferably 10 to 24 carbon atoms; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and 2-ethylhexanol. In some preferred embodiments the quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N selected from N,N-dimethyl ethanolamine, dimethyl oleylamine, dimethyloctadecylamine, dimethyloctadecylamine, hexadecyl dimethyl amine, N-oleyl diethanolamine and N-methyl-N,N-ditallowamine; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) a polymeric ester of itaconic acid. In some preferred embodiments the quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N selected from N,N-dimethyl ethanolamine, dimethyl oleylamine, dimethyloctadecylamine, dimethyloctadecylamine, hexadecyl dimethyl amine, N-oleyl diethanolamine and N-methyl-N,N-ditallowamine; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 6 to 30, preferably 6 to 24, carbon atoms. In some preferred embodiments the quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N selected from N,N-dimethyl ethanolamine, dimethyl oleylamine, dimethyloctadecylamine, dimethyloctadecylamine, hexadecyl dimethyl amine, N-oleyl diethanolamine and N-methyl-N,N-ditallowamine; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 4 to 10 carbon atoms. In some preferred embodiments the quaternary ammonium salt comprises cations Q+ which are the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N selected from N,N-dimethyl ethanolamine, dimethyl oleylamine, dimethyloctadecylamine, dimethyloctadecylamine, N-oleyl diethanolamine, hexadecyl dimethyl amine, N-oleyl diethanolamine and N-methyl-N,N-ditallowamine; and (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and comprises anions An- derived from (c) the polymerised reaction product of itaconic acid / anhydride and 2-ethylhexanol. In some preferred embodiments the or each quaternary ammonium salt is the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl or hydroxy alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or alkenyl group having 6 to 30, preferably 10 to 24 carbon atoms; (b) an epoxide selected from styrene oxide, ethylene oxide, propylene oxide, butylene oxide, stilbene oxide, dodecylene oxide 2-ethylhexyl glycidyl ether and isopropyl glycidyl ether; and (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 6 to 30, preferably 6 to 24, carbon atoms. In some preferred embodiments the or each quaternary ammonium salt is the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl or hydroxy alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or alkenyl group having 6 to 30, preferably 10 to 24 carbon atoms; (b) an epoxide selected from styrene oxide, ethylene oxide, propylene oxide, butylene oxide, stilbene oxide, dodecylene oxide 2-ethylhexyl glycidyl ether and isopropyl glycidyl ether; and (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 4 to 10 carbon atoms. In some preferred embodiments the or each quaternary ammonium salt is the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N selected from N,N-dimethyl ethanolamine, dimethyl oleylamine, N-oleyl diethanolamine dimethyloctadecylamine, dimethyloctadecylamine, N-oleyl diethanolamine, hexadecyl dimethyl amine, N-oleyl diethanolamine and N-methyl-N,N-ditallowamine; (b) an epoxide selected from styrene oxide, ethylene oxide, propylene oxide, butylene oxide, stilbene oxide, dodecylene oxide 2-ethylhexyl glycidyl ether and isopropyl glycidyl ether; and (c) the polymerised reaction product of itaconic acid / anhydride and 2-ethylhexanol. In some embodiments the or each quaternary ammonium salt is obtained by reacting: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl or hydroxy alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or alkenyl group having 6 to 30, preferably 10 to 24 carbon atoms; with (b) quaternising agent selected from dimethyl carbonate, methyl salicylate and dimethyl oxalate; and then carrying out an anion exchange reaction by reaction with (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 6 to 30, preferably 6 to 24, carbon atoms. In some embodiments the or each quaternary ammonium salt is obtained by reacting: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl or hydroxy alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or alkenyl group having 6 to 30, preferably 10 to 24 carbon atoms; with (b) quaternising agent selected from dimethyl carbonate, methyl salicylate and dimethyl oxalate; and then carrying out an anion exchange reaction by reaction with (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 4 to 10 carbon atoms. In some embodiments the quaternary ammonium salt is obtained by reacting: (a) a compound including a tertiary amino group of formula R5R6R7N selected from N,N-dimethyl ethanolamine, dimethyl oleylamine, N-oleyl diethanolamine dimethyloctadecylamine, dimethyloctadecylamine, N-oleyl diethanolamine, hexadecyl dimethyl amine, N-oleyl diethanolamine and N-methyl-N,N-ditallowamine; with (b) a quaternising agent selected from dimethyl carbonate, methyl salicylate and dimethyl oxalate and then carrying out an anion exchange reaction with (c) the polymerised reaction product of itaconic acid / anhydride and 2-ethylhexanol. The additive composition of the first aspect of the present invention comprises at least one quaternary ammonium salt comprising cations Q+ and anions An-. Preferably the additive composition further comprises a diluent or carrier. Suitable diluents and carriers will be known to the person skilled in the art. Aromatic diluents are preferred. The additive composition of the first aspect may be an additive composition for lubricating oil. The additive composition of the first aspect may be an additive composition for gasoline. Preferably the additive composition of first aspect is an additive composition for diesel fuel. The quaternary ammonium salt comprising cations Q+ is suitably present in the additive composition in an amount of from 1 to 99 wt%, for example from 1 to 75 wt%. The additive composition may comprise a mixture of two or more quaternary ammonium salts comprising cations Q+ and anions An-. In such embodiments the above amounts suitably refer to the total amount of all such compounds present in the composition. The additive composition may include one or more further additives. These may be selected from antioxidants, dispersants, detergents, metal deactivating compounds, wax anti-settling agents, cold flow improvers, cetane improvers, dehazers, stabilisers, demulsifiers, antifoams, corrosion inhibitors, lubricity improvers, dyes, markers, combustion improvers, metal deactivators, odour masks, drag reducers and conductivity improvers. In some preferred embodiments the additive composition includes one or more further nitrogen-containing detergents. These are suitably as defined later herein in relation to the third aspect. The present invention provides the use of at least one quaternary ammonium salt comprising cations Q+ and anions An- as an additive for fuel or lubricating oil compositions. The present invention may provide the use of at least one quaternary ammonium salt comprising cations Q+ and anions as a deposit control additive for fuel or lubricating oil compositions. The present invention may provide the use of at least one quaternary ammonium salt comprising cations Q+ and anions An- as a deposit control additive for lubricating oil compositions. The present invention may provide the use of at least one quaternary ammonium salt comprising cations Q+ and anions An- as a deposit control additive for fuel compositions. The present invention may provide the use of at least one quaternary ammonium salt comprising cations Q+ and anions An- as a deposit control additive for gasoline or diesel fuel compositions. The present invention may provide the use of at least one quaternary ammonium salt comprising cations Q+ and anions An- as a deposit control additive for gasoline fuel compositions. The present invention may provide the use of at least one quaternary ammonium salt comprising cations Q+ and anions An- as a deposit control additive for diesel fuel compositions. The present invention may provide a fuel or lubricating oil composition comprising at least one quaternary ammonium salt comprising cations Q+ and anions An-. According to a second aspect of the present invention there is provided a lubricating composition comprising an oil of lubricating viscosity and as an additive at least one quaternary ammonium salt comprising cations Q+ and anions An-; wherein Q+ is a quaternary ammonium cation; An- is an itaconic acid based anion; and n is at least 1. Preferred features of the quaternary ammonium salt are as defined in relation to the first aspect. The additive composition of the first aspect suitably upon dilution provides a lubricating composition of the second aspect. According to a third aspect of the present invention there is provided a fuel composition comprising as an additive at least one quaternary ammonium salt comprising cations Q+ and anions An-; wherein Q+ is a quaternary ammonium cation; An- is an itaconic acid based anion; and n is at least 1. Preferred features of the quaternary ammonium salt are as defined in relation to the first aspect. The additive composition of the first aspect suitably upon dilution provides a fuel composition of the third aspect. Additives of the invention may be added to diesel fuel at any convenient place in the supply chain. For examples, the additives may be added to fuel at the refinery, at a distribution terminal or after the fuel has left the distribution terminal. If the additive is added to the fuel after it has left the distribution terminal, this is termed an aftermarket application. Aftermarket applications include such circumstances as adding the additive to the fuel in the delivery tanker, directly to a customer’s bulk storage tank, or directly to the end user’s vehicle tank. Aftermarket applications may include supplying the fuel additive in small bottles suitable for direct addition to fuel storage tanks or vehicle tanks. The present invention may further provide a method of preparing a fuel composition, the method comprising: - preparing a quaternary ammonium compound by reacting (a) a compound including a tertiary amino group with (b) a quaternising agent either in the presence of or followed by an anion exchange reaction with (c) itaconic acid or an ester and / or polymer thereof; and - mixing the quaternary ammonium compound into the fuel. The fuel composition of the present invention may be a gasoline composition or a diesel fuel composition. Preferably it is a diesel fuel composition. By diesel fuel we include 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, 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). The diesel fuel composition may comprise a renewable fuel such as a biofuel composition or biodiesel composition. The diesel fuel composition may comprise first generation biodiesel. First generation biodiesel contains esters of, for example, vegetable oils, animal fats and used cooking fats or oils. This form of biodiesel may be obtained by transesterification of oils, for example rapeseed oil, soybean oil, canola oil, safflower oil, palm oil, corn oil, peanut oil, cotton seed oil, tallow, coconut oil, physic nut oil (Jatropha), sunflower seed oil, used cooking oils, hydrogenated vegetable oils or any mixture thereof, with an alcohol, usually a monoalcohol, usually in the presence of a catalyst. The diesel fuel composition may comprise second generation biodiesel. Second generation biodiesel is derived from renewable resources such as vegetable oils and animal fats and processed, often in the refinery, using, for example, hydroprocessing such as the H-Bio process developed by Petrobras. Second generation biodiesel may be similar in properties and quality to petroleum based fuel oil streams, for example renewable diesel produced from vegetable oils, animal fats etc. and marketed by ConocoPhillips as Renewable Diesel and by Neste as NExBTL. The diesel fuel composition may comprise 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, but aims to exploit the whole plant (biomass) and thereby widens the feedstock base. In some embodiments the diesel fuel composition may comprise a pyrolysis oil, for example a plastic pyrolysis oil ora biomass (wood, vegetable oil, algae) pyrolysis oil. The diesel fuel composition 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 bio-diesel. In such blends the bio-diesel may be present in an amount of, 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 biodiesel. In some preferred embodiments the fuel composition comprises at least 5 wt% biodiesel. In some embodiments the fuel composition may comprise GTL fuel or be a neat GTL fuel. In some embodiments the diesel fuel composition may comprise a secondary fuel, for example ethanol. Preferably however the diesel fuel composition does not contain ethanol. The diesel fuel composition used in the present invention may contain a relatively high sulphur content, for example greater than 0.05% by weight, such as 0.1% or 0.2%. However, in preferred embodiments the diesel fuel composition has a sulphur content of at most 0.05% by weight, more preferably of at most 0.035% by weight, especially of at most 0.015%. Fuels with even lower levels of sulphur are also suitable such as, fuels with less than 50 ppm sulphur by weight, preferably less than 20 ppm, for example 10 ppm or less. In some embodiments the diesel fuel composition used in the present invention preferably comprises at least 5 wt% biodiesel and less than 50 ppm sulphur. In some embodiments the diesel fuel composition comprises about 7 wt% biodiesel. In some embodiments the diesel and composition comprises about 10 wt% biodiesel. In some embodiments the diesel fuel composition is neat renewable diesel. The diesel fuel composition may be suitably 100% derived from renewable sources. Such a fuel is referred to herein as a renewable diesel. A suitable renewable diesel is a renewable diesel obtained by the hydrodeoxygenation of fats and oils (the fats and oils being derived from renewable sources). For example, the renewable diesel may be a hydrotreated triglyceride oil such as a hydrogenated vegetable oil (HVO). The HVO suitably complies with EN15940 Class A. Such HVO fuels are available from Coryton and Neste. Such renewable diesel fuels are suitably 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 renewable diesel 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 renewable diesel 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 renewable diesel 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 mm2s'1, most preferably from 2 to 4.5 mm2s' 1. Kinematic viscosity may be measured according to ASTM D445. Preferably the renewable diesel 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 renewable diesel 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. In preferred embodiments the renewable diesel comprises predominately straight chain alkanes and branched alkanes. Preferably the renewable diesel 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 naphthene 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 renewable diesel 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. 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 renewable diesel suitably has an oxygen content of less than 1wt%, preferably less than 0.1 wt% when measured according to EN 14078. The renewable diesel suitably has an aromatic content of less than 5 wt%, preferably less than 1 wt%. Preferably the renewable diesel is a hydrotreated triglyceride oil, suitably an HVO, having the contents and properties described above. Suitably the renewable diesel complies with the standard specification set out in EN15940. Preferably the renewable diesel is a hydrotreated triglyceride oil, suitably an HVO, having an aromatic content of less than 5 wt%, preferably less than 1 wt%. The diesel fuel composition used in the present invention may comprise at least 10vol% renewable diesel as defined above, suitably at least 25 vol%, at least 50 vol% or at least 90 vol% renewable diesel, suitably wherein the renewable diesel is hydrotreated triglyceride oil, preferably HVO. 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 fuel composition of the present invention comprises renewable diesel and one or more further components selected from biodiesel, mineral diesel and mixtures thereof. In some embodiments the fuel composition comprises renewable diesel and at least 5 vol% biodiesel. In some embodiments the fuel composition comprises renewable diesel and at least 20 vol% biodiesel. In some embodiments the fuel composition comprises 80 vol% renewable diesel and 20 vol% biodiesel. In preferred embodiments the diesel fuel composition is neat (i.e. 100 vol%) renewable diesel as defined above, preferably neat hydrotreated triglyceride oil, preferably neat HVO. Such a fuel may be referred to as an R100 diesel fuel. Various metal species may be present in the diesel fuel composition. This may be due to contamination of the fuel during manufacture, storage, transport or use or due to contamination of fuel additives. Metal species may also be added to fuels deliberately. For example, transition metals are sometimes added as fuel borne catalysts, for example to improve the performance of diesel particulate filters. Other metal-containing species may also be present as a contaminant, for example through the corrosion of metal and metal oxide surfaces by acidic species present in the fuel or from lubricating oil. In use, fuels such as diesel fuels routinely come into contact with metal surfaces for example, in vehicle fuelling systems, fuel tanks, fuel transportation means etc. Typically, metal-containing contamination may comprise transition metals such as zinc, iron and copper; Group I or Group II metals and other metals such as lead. Typically, the total amount of all metal-containing species in the diesel fuel, expressed in terms of the total weight of metal in the species, is between 0.1 and 50 ppm by weight, for example between 0.1 and 20 ppm, preferably between 0.1 and 10 ppm by weight, based on the weight of the diesel fuel. Suitably the quaternary ammonium salt comprising cations Q+ and anions An- present in the diesel fuel composition in an amount of at least 0.1 ppm, preferably at least 1 ppm, more preferably at least 5 ppm, suitably at least 10 ppm, for example at least 20 ppm or at least 25 ppm. Suitably the quaternary ammonium salt comprising cations Q+ and anions An- is present in the diesel fuel composition in an amount of less than 10000 ppm, preferably less than 1000 ppm, preferably less than 500 ppm, preferably less than 250 ppm, suitably less than 200 ppm, for example less than 150 ppm or less than 100 ppm. Preferably the quaternary ammonium salt comprising cations Q+ and anions An- is present in the fuel composition in an amount of from 0.1 to 1000 ppm, preferably from 1 to 500 ppm, suitably from 5 to 250 ppm, for example from 10 to 100 ppm. For the avoidance of doubt, references herein to ppm, unless otherwise stated, are to parts per million by weight. The diesel fuel composition of the third aspect of the present invention may comprise a mixture of two or more quaternary ammonium salts comprising cations Q+ and anions An-. In such embodiments the above amounts refer to the total amounts of all such additives present in the composition. The amounts above relate to the amount of active quaternary ammonium salt and do not include any diluent, solvent or carrier or any residual starting material, impurities or byproducts which may be present. The diesel fuel composition of the present invention may include one or more further additives such as those which are commonly found in diesel fuels. These include, for example, antioxidants, dispersants, detergents, metal deactivating compounds, wax anti-settling agents, cold flow improvers, cetane improvers, dehazers, stabilisers, demulsifiers, antifoams, corrosion inhibitors, lubricity improvers, dyes, markers, combustion improvers, metal deactivators, odour masks, drag reducers and conductivity improvers. Examples of suitable amounts of each of these types of additives will be known to the person skilled in the art. In some preferred embodiments the diesel fuel composition of the present invention comprises one or more further detergents. Nitrogen-containing detergents are preferred. The one or more further detergents may be selected from: (I) a quaternary ammonium salt additive which is not a quaternary ammonium compound as defined in relation to the first aspect; (II) the product of a Mannich reaction between an aldehyde, an amine and an optionally substituted phenol; (III) the reaction product of a carboxylic acid-derived acylating agent and an amine; (IV) the reaction product of a hydrocarbyl-substituted dicarboxylic acid or anhydride and an amine compound or salt which product comprises at least one amino triazole group; (V) partial esters of substituted succinic acids. Preferably one or more further detergents are selected from one or more of: (I) a quaternary ammonium salt additive which is not a quaternary ammonium compound of the first aspect; (II) the product of a Mannich reaction between an aldehyde, an amine and an optionally substituted phenol; and (III) the reaction product of a carboxylic acid-derived acylating agent and an amine. The weight ratio of the additive of the present invention to the nitrogen containing detergent is suitably from 10:1 to 1:10, preferably 5:1 to 1:5, preferably from 2:1 to 1:2. In some embodiments the diesel fuel composition further comprises (I) a quaternary ammonium salt additive which is not a quaternary ammonium compound of the first aspect. Suitably the additional quaternary ammonium salt does not include an anion which is derived from itaconic acid or an ester and / or polymer thereof. The quaternary ammonium salt additive is suitably the reaction product of a nitrogencontaining species having at least one tertiary amine group and a quaternising agent. The nitrogen containing species having at least one tertiary amine group may be selected from any of components (i) to (vi) defined in relation to component (a) used to prepared the quaternary ammonium salt comprising cations Q+ and anions An-defined in relation to the first aspect. In preferred embodiments the nitrogen containing species having at least one tertiary amine group is (i) the reaction product of a hydrocarbyl-substituted acylating agent and a compound having at least one tertiary amine group and a primary amine, secondary amine or alcohol group. To form the quaternary ammonium salt additive (I) the nitrogen-containing species having a tertiary amine group is reacted with a quaternising agent. The quaternising agent may suitably be selected from esters and non-esters. Preferred quaternising agents for use herein include dimethyl oxalate, methyl 2-nitrobenzoate, methyl salicylate and styrene oxide or propylene oxide optionally in combination with an additional acid. An especially preferred additional quaternary ammonium salt for use herein is formed by reacting methyl salicylate or dimethyl oxalate with the reaction product of a polyisobutylenesubstituted succinic anhydride having a PIB number average molecular weight of 700 to 1300 and dimethylaminopropylamine. Other suitable quaternary ammonium salts include quaternised terpolymers, for example as described in US2011 / 0258917; quaternised copolymers, for example as described in US2011 / 0315107; and the acid-free quaternised nitrogen compounds disclosed in US2012 / 0010112. Further suitable quaternary ammonium compounds for use as additional quaternary ammonium salts (I) include the quaternary ammonium compounds described in the applicants copending applications WO2011095819, WO2013 / 017889, WO2015 / 011506, WO2015 / 011507, WO2016 / 016641 and PCT / GB2016 / 052312. In some especially preferred embodiments the diesel fuel composition comprises an additional quaternary ammonium salt additive (la) which is the quaternised reaction product of a hydrocarbyl substituted succinic acid derived acylating agent and a compound able to react with said acylating agent and which includes a tertiary amine group; wherein each molecule of the hydrocarbyl substituted succinic acid derived acylating agent includes on average at least 1.2 succinic acid moieties. Preferred hydrocarbyl-based substituents are polyisobutenes. Such compounds are known to the person skilled in the art. Preferred hydrocarbyl substituted succinic acid derived acylating agents for use in preparing additive (la) are polyisobutenyl substituted succinic anhydrides or PIBSAs. Especially preferred PIBSAs are those having a PIB molecular weight (Mn) of from 300 to 2800, preferably from 450 to 2300, more preferably from 500 to 1300. In preferred embodiments the further quaternary ammonium salts are derived from hydrocarbyl substituted acylating agents which include an average of at least 1.2 succinic acid moieties per molecule. Preferably the hydrocarbyl substituted succinic acid derived acylating agent comprises on average at least 1.21 succinic acid moieties per molecule, more preferably at least 1.22 succinic acid moieties per molecule. In some embodiments the hydrocarbyl substituted succinic acid derived acylating agent may comprise at least 1.23 or at least 1.24 succinic acid moieties per molecule. In some embodiments the hydrocarbyl substituted succinic acid derived acylating agent may comprise at least 1.25, at least 1.26 or at least 1.27 succinic acid moieties per molecule. In some embodiments the hydrocarbyl substituted succinic acid derived acylating agent may comprise at least 1.28, at least 1.29 or at least 1.30 succinic acid moieties per molecule. By succinic acid moiety we mean to include residues of succinic acid present in diacid or anhydride form. In some embodiments the additional quaternary ammonium compounds are the quaternised reaction product of a fatty acid (for example oleic acid) and dimethylaminopropyl amine. The hydrocarbyl substituted succinic acid derived acylating agent is reacted with a compound able to react with said acylating agent and which includes a tertiary amine group. The tertiary amine group is quaternised to provide the quaternary ammonium salt additive. Preferably the compound able to react with hydrocarbyl substituted succinic acid derived acylating agent and which includes a tertiary amine group is an amine of formula (7) or (8) as previously defined in relation to quaternary ammonium salt used in the first aspect comprising cations Q+ and anions An-. Preferred compounds of formula (7) include N,N-dimethyl-1,3-diaminopropane, N,N-diethyl-1,3- diaminopropane, N,N-dimethylethylenediamine, N,N-diethylethylenediamine, N,N-dibutylethylenediamine, 3-(2-(dimethylamino)ethoxy) propylamine, or combinations thereof. Preferred compounds of formula (8) include Triisopropanolamine, 1-[2-hydroxyethyl]piperidine, 2-[2-(dimethylamine)ethoxy]-ethanol, N-ethyldiethanolamine, N-methyldiethanolamine, N-butyldiethanolamine, N,N-diethylaminoethanol, N,N-dimethylaminoethanol, 2-dimethylamino-2-methyl-1-propanol, or combinations thereof. An especially preferred compound of formula (7) is N,N-dimethyl-1,3-diaminopropane (dimethylaminopropylamine). When a compound of formula (8) is reacted with a succinic acylating agent the resulting product is a succinic ester. When a succinic acylating agent is reacted with a compound of formula (7) in which R4 is hydrogen the resulting product may be a succinimide or a succinamide. When a succinic acylating agent is reacted with a compound of formula (7) in which R4 is not hydrogen the resulting product is an amide. To form the additional quaternary ammonium salt additive (la) the hydrocarbyl substituted succinic acid derived acylating agent is reacted with a compound able to react with said acylating agent and which includes a tertiary amine group. This reaction product is then quaternised by reaction with a quaternising agent. The reaction product of the acylating agent and compound which includes a tertiary amine group is preferably reacted with at least one molar equivalent of quaternising agent per mole of tertiary amine group present in the reaction product. Preferably the reaction product of the acylating agent and compound which includes a tertiary amine group is reacted with more than one molar equivalent of quaternising agent per mole of tertiary amine group present in the reaction product, preferably at least 1.2 molar equivalents of quaternising agent per mole of tertiary amine group, more preferably at least 1.5 molar equivalents of quaternising agent, suitably at least 1.7 molar equivalents of quaternising agent, for example at least 1.9 molar equivalents of quaternising agent. Preferably the reaction product of the acylating agent and compound which includes a tertiary amine group is reacted with two or more molar equivalents of quaternising agent per mole of tertiary amine group present in the reaction product, preferably at least 2.1 molar equivalents of quaternising agent. In some embodiments the reaction product of the acylating agent and compound which includes a tertiary amine group is reacted with more than 2.2 molar equivalents of quaternising agent per mole of tertiary amine group present in the reaction product, for example from 2.3 to 4 molar equivalents, from 2.3 to 3 molar equivalents, or from 2.3 to 2.7 or from 2.5 to 3 molar equivalents. Any suitable quaternising agent may be used. The quaternising agent may suitably be selected from esters and non-esters. Suitable quaternising agents include esters of a carboxylic acid, dialkyl sulfates, benzyl halides, hydrocarbyl substituted carbonates, hydrocarbyl substituted epoxides optionally in combination with an acid, alkyl halides, alkyl sulfonates, sulfones, hydrocarbyl substituted phosphates, hydrocarbyl substituted borates, alkyl nitrites, alkyl nitrates, hydroxides, N-oxides, chloroacetic acid or salts thereof, or mixtures thereof. In some preferred embodiments, quaternising agents used to form the additional quaternary ammonium salt additives are esters. Suitable ester quaternising agents are as defined in relation to the quaternary ammonium salt of the first aspect comprising cations Q+ and anions An-. Preferred ester quaternising agents include dimethyl oxalate, methyl 2-nitrobenozoate and methyl salicylate. Dimethyl oxalate and methyl salicylate are especially preferred. Suitable non-ester quaternising agents include dialkyl sulfates, benzyl halides, hydrocarbyl substituted carbonates, hydrocarbyl substituted epoxides optionally in combination with an acid, alkyl halides, alkyl sulfonates, sulfones, hydrocarbyl substituted phosphates, hydrocarbyl substituted borates, alkyl nitrites, alkyl nitrates, hydroxides, N-oxides, chloroacetic acid or salts thereof, or mixtures thereof. Suitable hydrocarbyl substituted epoxides are as described in relation to component (b) used to prepare the quaternary ammonium salts used in the first aspect comprising cations Q+ and anions An-. The hydrocarbyl epoxides are used as quaternising agents optionally in combination with an acid. Hydrocarbyl substituted succinic acylating agents include two acyl groups. In some embodiments only one of these groups reacts with the compound of formula (7) or formula (8) to form a compound having an ester or an amide functional group and a free carboxylic acid. In these embodiments if an epoxide is used as the quaternising agent, a separate acid does not need to be added. However in other embodiments an acid for example acetic acid may be used. Especially preferred epoxide quaternising agents are propylene oxide and styrene oxide, optionally in combination with an additional acid. Preferred quaternising agents for use in preparing the additional quaternary ammonium salt additives include dimethyl oxalate, methyl 2-nitrobenzoate, methyl salicylate, chloroacetic acid or a salt thereof, and styrene oxide or propylene oxide optionally in combination with an additional acid. In some embodiments mixtures of two or more quaternising agents may be used. To form some preferred quaternary ammonium salt additives (la) for use in the present invention a compound of formula (21) is reacted with a compound formed by the reaction of a hydrocarbyl substituted succinic acid acylating agent and an amine of formula (7) or (8). Suitably approximately one equivalent of amine is added per succinic acid moiety present in the acylating agent. The ratio of amine used will thus typically depend on the average number of succinic acid moieties present in each molecule of the acylating agent. Suitable additional quaternary ammonium salts (I) which may be present in the fuel composition of the present invention include the reaction product of: (x) a polyisobutenyl substituted succinic acid or anhydride thereof having a PIB molecular weight of 170 to 2800, preferably 450 to 1500 and an amine or alcohol selected from dimethylaminopropanol and dimethylaminopropylamine; and (y) a quaternising agent selected from dimethyl oxalate; methyl salicylate; and an epoxide selected from styrene oxide, propylene oxide and butylene oxide, optionally in combination with an acid. Preferred additional quaternary ammonium salts (I) which may be present in the fuel composition of the present invention include the reaction product of: (x) a polyisobutenyl substituted succinic acid or anhydride thereof having a PIB molecular weight of 170 to 2800, preferably 450 to 1500 and including an average of at least 1.2 succinic acid moieties per molecule, and an amine or alcohol selected from dimethylaminopropanol and dimethylaminopropylamine; and (y) a quaternising agent selected from dimethyl oxalate; methyl salicylate; and an epoxide selected from styrene oxide, propylene oxide and butylene oxide, in combination with an acid. An especially preferred additional quaternary ammonium salt (la) for use herein is formed by reacting methyl salicylate or dimethyl oxalate with the reaction product of a polyisobutylenesubstituted succinic anhydride having a PIB molecular weight of 700 to 1300 and dimethylaminopropylamine; wherein the polyisobutylene-substituted succinic anhydride includes on average at least 1.2 succinic acid moieties per molecule. In some embodiments the additional quaternary ammonium salt (I) may comprise the quaternised reaction product of a fatty acid (for example oleic acid) and dimethylaminopropyl amine. For example the additional quaternary ammonium salt (I) may comprise the reaction product of oleic acid or a reactive equivalent thereof and dimethylaminopropyl amine quaternised by reaction with chloroacetic acid or a salt thereof. In some embodiments the additional quaternary ammonium salt (I) may comprise a quaternary ammonium compound which is the reaction product of a tertiary amine of formula R5R6R7N, wherein one or two of the groups R5, R6 and R7 is a short chain alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or group having 6 to 30, preferably 10 to 24 carbon atoms; an epoxide; and a monocarboxylic acid or a dicarboxylic acid. In some embodiments the additional quaternary ammonium salt (I) may comprise a quaternary ammonium compound which is the reaction product of a tertiary amine; an epoxide, preferably propylene oxide; and an optionally substituted succinic acid, preferably a polyisobutenyl substituted succinic acid wherein the tertiary amine has the formula R5R6R7N, wherein one or two of the groups R5, R6 and R7 is a short chain alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or group having 6 to 30, preferably 10 to 24 carbon atoms. In some embodiments the additional quaternary ammonium salt (I) may comprise a quaternary ammonium compound which is the reaction product of dimethylhexadecylamine, propylene oxide and an acid, preferably a polyisobutenyl substituted succinic acid. In preferred embodiments the succinic acid has a polyisobutenyl substituent with a number average molecular weight of 450 to 1500. In some embodiments the diesel fuel composition used in the present invention comprises from 1 to 500 ppm, preferably 5 to 250 ppm of a quaternary ammonium salt comprising cations Q+ and anions An- defined in relation to the first aspect and from 1 to 500 ppm, preferably 5 to 250 ppm of an additional quaternary ammonium salt (I), preferably a quaternary ammonium salt (la). In some embodiments the diesel fuel composition comprises further (II) the product of a Mannich reaction between an aldehyde, an amine and an optionally substituted phenol. This Mannich reaction product is suitably not a quaternary ammonium salt. Preferably the aldehyde component used to prepare the Mannich additive is an aliphatic aldehyde. Preferably the aldehyde has 1 to 10 carbon atoms. Most preferably the aldehyde is formaldehyde. Suitable amines for use in preparing the Mannich additive include monoamines and polyamines. One suitable monoamine is butylamine. The amine used to prepare the Mannich additive is preferably a polyamine. This may be selected from any compound including two or more amine groups. Preferably the polyamine is a polyalkylene polyamine, preferably a polyethylene polyamine. Most preferably the polyamine comprises tetraethylenepentamine or ethylenediamine. The optionally substituted phenol component used to prepare the Mannich additive is preferably a mono-substituted phenol, preferably a hydrocarbyl substituted phenol. Preferred hydrocarbyl substituents are alkyl substituents having 4 to 28 carbon atoms, especially 10 to 14 carbon atoms. Other preferred hydrocarbyl substituents are polyalkenyl substituents. Such polyisobutenyl substituents having a number average molecular weight of from 400 to 2500, for example from 500 to 1500. In some embodiments the diesel fuel composition further comprises (III) the reaction product of a carboxylic acid-derived acylating agent and an amine. These may also be referred to herein in general as acylated nitrogen-containing compounds. Suitable acylated nitrogen-containing compounds may be made by reacting a carboxylic acid acylating agent with an amine and are known to those skilled in the art. Preferred hydrocarbyl substituted acylating agents are polyisobutenyl succinic anhydrides. These compounds are commonly referred to as “PIBSAs” and are known to the person skilled in the art. Conventional polyisobutenes and so-called “highly-reactive” polyisobutenes are suitable for use in preparing the additional detergent additives (III). Especially preferred PIBSAs are those having a PIB molecular weight (Mn) of from 300 to 2800, preferably from 450 to 2300, more preferably from 500 to 1300. In preferred embodiments the reaction product of the carboxylic acid derived acylating agent and an amine includes at least one primary or secondary amine group. A preferred acylated nitrogen-containing compound for use herein is prepared by reacting a poly(isobutene)-substituted succinic acid-derived acylating agent (e.g., anhydride, acid, ester, etc.) wherein the poly(isobutene) substituent has a number average molecular weight (Mn) of between 170 to 2800 with a mixture of ethylene polyamines having 2 to about 9 amino nitrogen atoms, preferably about 2 to about 8 nitrogen atoms, per ethylene polyamine and about 1 to about 8 ethylene groups. These acylated nitrogen compounds are suitably formed by the reaction of a molar ratio of acylating agent:amino compound of from 10:1 to 1:10, preferably from 5:1 to 1:5, more preferably from 2:1 to 1:2 and most preferably from 2:1 to 1:1. In especially preferred embodiments, the acylated nitrogen compounds are formed by the reaction of acylating agent to amino compound in a molar ratio of from 1.8:1 to 1:1.2, preferably from 1.6:1 to 1:1.2, more preferably from 1.4:1 to 1:1.1 and most preferably from 1.2:1 to 1:1. Acylated amino compounds of this type and their preparation are well known to those skilled in the art and are described in for example EP0565285 and US5925151. In some preferred embodiments the composition comprises a detergent of the type formed by the reaction of a polyisobutene-substituted succinic acid-derived acylating agent and a polyethylene polyamine. Suitable compounds are, for example, described in WO2009 / 040583. In some embodiments the diesel fuel composition further comprises (IV) the reaction product of a hydrocarbyl-substituted dicarboxylic acid or anhydride and an amine compound or salt which product comprises at least one amino triazole group. Further preferred features of additive compounds of this type are as defined in US2009 / 0282731. In some embodiments the diesel fuel composition further comprises (V) a partial ester of a substituted succinic acid. Preferred compounds of this type are ester compounds which are the reaction product of a hydrocarbyl substituted succinic acid or a hydrocarbyl substituted succinic anhydride.and an alcohol or formula H-(OR22)n-OR21, wherein R is an optionally substituted alkylene group; R21 is hydrogen or an optionally substituted hydrocarbyl group, and n is 0 or a positive integer; wherein n is not 0 when R21 is hydrogen. Further preferred features of these detergents are described in WO2018 / 178680, WO2018 / 178678, WO2018 / 178695 and WO2018 / 178674. In some especially preferred embodiments the third aspect of the present invention relates to a diesel fuel composition comprising: - at least one quaternary ammonium salt comprising cations Q+ and anions An-; wherein Q+ is a quaternary ammonium cation and An- is an itaconic acid based anion; and - an additional further quaternary ammonium salt additive. Preferred additional quaternary ammonium salt additives are the reaction product of a nitrogen-containing species having at least one tertiary amine group and a quaternising agent wherein the nitrogen containing species is the reaction product of a hydrocarbyl-substituted acylating agent and a compound comprising at least one tertiary amine group and a primary amine, secondary amine or alcohol group. Preferably the quaternising agent is an ester quaternising agent. In some preferred embodiments of the third aspect of the present invention there is provided a diesel fuel composition comprising: - a quaternary ammonium salt comprising cations Q+ and anions An- which are the reaction product of: (a) a compound including a tertiary amino group which is the reaction product of a hydrocarbyl-substituted acylating agent and a compound having at least one tertiary amine group and a primary amine, secondary amine or alcohol group; or a compound including a tertiary amino group of formula R5R6R7N, wherein each of R5, R6 and R7 is independently an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group; (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and anions An- derived from (c) a polymeric ester of itaconic acid; and - an additional quaternary ammonium salt additive. The additional quaternary ammonium salt additive is preferably the quaternised reaction product of a hydrocarbyl substituted succinic acid derived acylating agent and a compound able to react with said acylating agent and which includes a tertiary amine group. Preferably each molecule of the hydrocarbyl substituted succinic acid derived acylating agent includes on average at least 1.2 succinic acid moieties. In some preferred embodiments of the third aspect of the present invention there is provided a diesel fuel composition comprising: - a quaternary ammonium salt which is the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or group having 6 to 30, preferably 10 to 24 carbon atoms; (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 6 to 30, preferably 6 to 24, carbon atoms; and - an additional quaternary ammonium salt additive. In some preferred embodiments of the third aspect of the present invention there is provided a diesel fuel composition comprising: - a quaternary ammonium salt which is the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or group having 6 to 30, preferably 10 to 24 carbon atoms; (b) a quaternising agent selected from epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl (preferably branched) group having 4 to 10 carbon atoms; and - an additional quaternary ammonium salt additive. The additional quaternary ammonium salt additive is preferably the quaternised reaction product of a hydrocarbyl substituted succinic acid derived acylating agent and a compound able to react with said acylating agent and which includes a tertiary amine group. Preferably each molecule of the hydrocarbyl substituted succinic acid derived acylating agent includes on average at least 1.2 succinic acid moieties. In some preferred embodiments of the third aspect of the present invention there is provided a diesel fuel composition comprising: - a quaternary ammonium compound which is the reaction product of: (a) a compound including a tertiary amino group of formula R5R6R7N selected from N,N-dimethyl ethanolamine, dimethyl oleylamine, dimethyloctadecylamine and N-methyl-N,N-ditallowamine; (b) an epoxide selected from styrene oxide, ethylene oxide, propylene oxide, butylene oxide, stilbene oxide, dodecylene oxide 2-ethylhexyl glycidyl ether and isopropyl glycidyl ether; and (c) the polymerised reaction product of itaconic acid / anhydride and 2-ethylhexanol; and - an additional quaternary ammonium salt additive which is the reaction product of: (x) a polyisobutenyl substituted succinic acid or anhydride thereof having a PIB molecular weight of 170 to 2800, preferably 450 to 1500 and including an average of at least 1.2 succinic acid moieties per molecule, and an amine or alcohol selected from dimethylaminopropanol and dimethylaminopropylamine; and (y) a quaternising agent selected from dimethyl oxalate; methyl salicylate; and an epoxide selected from styrene oxide, propylene oxide and butylene oxide, in combination with an acid. In some preferred embodiments of the third aspect of the present invention there is provided a diesel fuel composition comprising: - a quaternary ammonium salt obtained by reacting: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl or hydroxy alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or alkenyl group having 6 to 30, preferably 10 to 24 carbon atoms; with (b) quaternising agent selected from dimethyl carbonate, methyl salicylate and dimethyl oxalate; and then carrying out an anion exchange reaction by reaction with (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl group having 6 to 30, preferably 6 to 24, carbon atoms; and - an additional quaternary ammonium salt additive. In some preferred embodiments of the third aspect of the present invention there is provided a diesel fuel composition comprising: - a quaternary ammonium salt obtained by reacting: (a) a compound including a tertiary amino group of formula R5R6R7N wherein each of R5, R6 and R7 is an alkyl group or a hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms; or wherein one or two of the groups R5, R6 and R7 is a short chain alkyl or hydroxy alkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or alkenyl group having 6 to 30, preferably 10 to 24 carbon atoms; with (b) quaternising agent selected from dimethyl carbonate, methyl salicylate and dimethyl oxalate; and then carrying out an anion exchange reaction by reaction with (c) the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is an optionally substituted alkyl or alkenyl (preferably branched) group having 4 to 10 carbon atoms; and - an additional quaternary ammonium salt additive. The additional quaternary ammonium salt additive is preferably the quaternised reaction product of a hydrocarbyl substituted succinic acid derived acylating agent and a compound able to react with said acylating agent and which includes a tertiary amine group. Preferably each molecule of the hydrocarbyl substituted succinic acid derived acylating agent includes on average at least 1.2 succinic acid moieties. In some preferred embodiments of the third aspect of the present invention there is provided a diesel fuel composition comprising: - a quaternary ammonium salt obtained by reacting: (a) a compound including a tertiary amino group of formula R5R6R7N selected from N,N-dimethyl ethanolamine, dimethyl oleylamine, N-oleyl diethanolamine dimethyloctadecylamine and N-methyl-N,N-ditallowamine; with (b) a quaternising agent selected from dimethyl carbonate, methyl salicylate and dimethyl oxalate; and then carrying out an anion exchange reaction with (c) the polymerised reaction product of itaconic acid / anhydride and 2-ethylhexanol; and - an additional quaternary ammonium salt additive which is the reaction product of: (x) a polyisobutenyl substituted succinic acid or anhydride thereof having a PIB molecular weight of 170 to 2800, preferably 450 to 1500 and including an average of at least 1.2 succinic acid moieties per molecule, and an amine or alcohol selected from dimethylaminopropanol and dimethylaminopropylamine; and (y) a quaternising agent selected from dimethyl oxalate; methyl salicylate; and an epoxide selected from styrene oxide, propylene oxide and butylene oxide, in combination with an acid. In some embodiments the fuel composition of the third aspect of the present invention is a gasoline composition. By the term "gasoline", it is meant a liquid fuel for use with spark ignition engines (typically or preferably containing primarily or only C4-C12 hydrocarbons) and satisfying international gasoline specifications, such as ASTM D-439 and EN228. The term includes blends of distillate hydrocarbon fuels with oxygenated components such as alcohols or ethers for example methanol, ethanol, butanol, methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE), as well as the distillate fuels themselves. In some embodiments the gasoline fuel composition of the present invention may comprise one or more further gasoline detergents. Additional gasoline detergents will be known to the person skilled in the art and are described, for example, in WO2023 / 111550. The gasoline composition may further comprise a carrier oil. The carrier oil may have any suitable molecular weight. A preferred molecular weight is in the range 500 to 5000. In one embodiment the carrier oil may comprise an oil of lubricating viscosity, including natural or synthetic oils of lubricating viscosity, oil derived from hydrocracking, hydrogenation, hydrofinishing, unrefined, refined and re-refined oils, or mixtures thereof. Natural oils include animal oils, vegetable oils, mineral oils or mixtures thereof. Synthetic oils may include hydrocarbon oils such as those produced by Fischer-Tropsch reactions and typically may be hydroisomerised Fischer-Tropsch hydrocarbons or waxes. In another embodiment the carrier oil may comprise a polyether carrier oil. In a preferred embodiment the polyether carrier oil is a mono end-capped polyalkylene glycol, especially a mono end-capped polypropylene glycol. Carrier oils of this type will be known to the person skilled in the art. The gasoline fuel compositions of the present invention may contain one or more further additives conventionally added to gasoline, for example other detergents, dispersants, antioxidants, anti-icing agents, metal deactivators, lubricity additives, friction modifiers, dehazers, corrosion inhibitors, dyes, markers, octane improvers, anti-valve-seat recession additives, stabilisers, demulsifiers, antifoams, odour masks, conductivity improvers and combustion improvers. The fuel compositions of the present invention can be used to provide improved performance in engines combusting said fuel compositions. According to a fourth aspect of the present invention there is provided a method of improving the performance of an engine, the method comprising combusting in said engine a fuel composition comprising as an additive at least one quaternary ammonium salt comprising cations Q+ and anions An-; wherein Q+ is a quaternary ammonium cation; An- is an itaconic acid based anion; and n is at least 1. According to a fifth aspect of the present invention there is provided the use of at least one additive in a fuel composition to improve the performance of an engine combusting said fuel composition wherein the or each additive is quaternary ammonium salt comprising cations Q+ and anions An-; wherein Q+ is a quaternary ammonium cation; An- is an itaconic acid based anion; and n is at least 1. Preferred features of the fourth and fifth aspects of the present invention are as defined in relation to the first, second and third aspects. Further preferred features of the invention will now be described. In the method of the fourth aspect the engine may be a gasoline engine and the fuel composition may be a gasoline fuel. Preferably in the method of the fourth aspect the engine is a diesel engine and the fuel composition is a diesel fuel composition. In the use of the fifth aspect the engine may be a gasoline engine and the fuel composition may be a gasoline fuel. Preferably in the use of the fifth aspect the engine is a diesel engine and the fuel composition is a diesel fuel composition. The method of the fourth aspect of the present invention improves the performance of an engine combusting a fuel composition. This improvement is suitably compared with the performance of an otherwise identical fuel which does not comprise the at least one quaternary ammonium salt comprising cations Q+ and anions An-. The method and use of the fourth and fifth aspects of the present invention are particularly effective at improving the performance of a modern diesel engine having a high pressure fuel system. Due to consumer demand and legislation, diesel engines have in recent years become much more energy efficient, show improved performance and have reduced emissions. A common problem with diesel engines is fouling of the injector, particularly the injector body, and the injector nozzle. Fouling may also occur in the fuel filter. Injector nozzle fouling occurs when the nozzle becomes blocked with deposits from the diesel fuel. Fouling of fuel filters may be related to the recirculation of fuel back to the fuel tank. Deposits increase with degradation of the fuel. Deposits may take the form of carbonaceous coke-like residues, lacquers or sticky or gum-like residues. Diesel fuels become more and more unstable the more they are heated, particularly if heated under pressure. Thus diesel engines having high pressure fuel systems may cause increased fuel degradation. In recent years the need to reduce emissions has led to the continual redesign of injection systems to help meet lower targets. This has led to increasingly complex injectors and lower tolerance to deposits. The problem of injector fouling may occur when using any type of diesel fuels. However, some fuels may be particularly prone to cause fouling or fouling may occur more quickly when these fuels are used. For example, fuels containing biodiesel and those containing metallic species may lead to increased deposits. When injectors become blocked or partially blocked, the delivery of fuel is less efficient and there is poor mixing of the fuel with the air. Over time this leads to a loss in power of the engine and increased exhaust emissions and poor fuel economy. Deposits are known to occur in the spray channels of the injector, leading to reduced flow and power loss. As the size of the injector nozzle hole is reduced, the relative impact of deposit build up becomes more significant. Deposits are also known to occur at the injector tip. Here they affect the fuel spray pattern and cause less effective combustion and associated higher emissions and increased fuel consumption. In addition to these “external” injector deposits in the nozzle hole and at the injector tip which lead to reduced flow and power loss, deposits may occur within the injector body causing further problems. These deposits may be referred to as internal diesel injector deposits (or IDIDs). IDIDs may occur further up inside the injector on the critical moving parts. They can hinder the movement of these parts affecting the timing and quantity of fuel injection. Since modern diesel engines operate under very precise conditions these deposits can have a significant impact on performance. IDIDs cause a number of problems, including power loss and reduced fuel economy due to less than optimal fuel metering and combustion. Initially the user may experience cold start problems and / or rough engine running. These deposits can lead to more serious injector sticking. This occurs when the deposits stop parts of the injector from moving and thus the injector stops working. When several or all of the injectors stick the engine may fail completely. IDIDs are recognised as a serious problem by those working in the field and a new engine test has been developed by the industry based organisation, the Coordinating European Council (CEC). The IDID DW10C test was developed to be able to discriminate between a fuel that produces no measurable deposits and one which produces deposits that cause startability issues considered unacceptable. The objective of the test is to discriminate between fuels that differ in their ability to produce IDIDs in direct injection common rail diesel engines. The test is still under development. However a suitable merit system which may be used to evaluate fuels in this specification is described in example 12. The present inventors have studied these internal diesel injector deposits and have found that they contain a number of components. However they believe that the presence of lacquers and / or carboxylate residues lead to injector sticking. Lacquers are varnish-like deposits which are insoluble in fuel and common organic solvents. Carboxylate residues may be present from a number of sources. By carboxylate residues we mean to refer to salts of carboxylic acids. These may be short chain carboxylic acids but more commonly long chain fatty acid residues are present. The carboxylic residues may be present as ammonium and / or metal salts. Both carboxylic acids and metals may be present in diesel fuel from a number of sources. Carboxylic acids are commonly added into fuel as lubricity additives and / or corrosion inhibitors; they may occur due to oxidation of the fuel and may form during the combustion process; residual fatty acids may be present in the fatty acid methyl esters included as biodiesel; and they may also be present as byproducts in other additives. Derivatives of fatty acids may also be present and these may react or decompose to form carboxylic acids. Various metals may be present in fuel compositions. This may be due to contamination of the fuel during manufacture, storage, transport or use or due to contamination of fuel additives. Metal species may also be added to fuels deliberately. The present inventors believe that one of the causes of injector sticking occurs when metal or ammonium species react with carboxylic acid species in the fuel. One example of injector sticking has arisen due to sodium contamination of the fuel. Sodium contamination may occur for example, due to sodium hydroxide used in a washing step in the hydrodesulfurisation process; or due to the use of sodium-containing corrosion inhibitors in pipelines. The presence of calcium may be from interaction with or contamination with a lubricant or from calcium chloride used in salt drying processes in refineries. Other metal contamination may occur for example during transportation due to water bottoms. Metal contamination of diesel fuel and the resultant formation of carboxylate salts is believed to be a major cause of injector sticking. The formation of lacquers is yet another major cause of injector sticking. Sodium contamination of diesel fuel and the resultant formation of carboxylate salts is believed to be a major cause of injector sticking. In some preferred embodiments the diesel fuel compositions used in the present invention comprise sodium and / or calcium. Preferably they comprise sodium. The sodium and / or calcium is typically present in a total amount of from 0.01 to 50 ppm, preferably from 0.05 to 5 ppm preferably 0.1 to 2ppm such as 0.1 to 1 ppm. The presence of metal containing species may give rise to fuel filter deposits and / or external injector deposits including injector tip deposits and / or nozzle deposits. In some embodiments, the diesel fuel may comprise metal-containing species comprising zinc. Zinc may be present in an amount of from 0.01 to 50 ppm, preferably from 0.05 to 5 ppm, more preferably 0.1 to 1.5 ppm. The method and use of the present invention provide an improvement in the performance of a diesel engine. This improvement in performance is suitably selected from one or more of: a reduction in power loss of the engine; a reduction in external diesel injector deposits; a reduction in internal diesel injector deposits; an improvement in fuel economy; a reduction in fuel filter deposits; a reduction in emissions; and an increase in maintenance intervals. Preferably the method and use of the present invention improve the performance of a modern diesel engine having a high pressure fuel system. Such diesel engines may be characterised in a number of ways. Such engines are typically equipped with fuel injection equipment meeting or exceeding “Euro 5” emissions legislation or equivalent legislation in US or other countries. Such engines are typically equipped with fuel injectors having a plurality of apertures, each aperture having an inlet and an outlet. Such engines may be characterised by apertures which are tapered such that the inlet diameter of the spray-holes is greater than the outlet diameter. Such modern engines may be characterised by apertures having an outlet diameter of less than 500pm, preferably less than 200pm, more preferably less than 150pm, preferably less than 100pm, most preferably less than 80pm or less. Such modern diesel engines may be characterised by apertures where an inner edge of the inlet is rounded. Such modern diesel engines may be characterised by the injector having more than one aperture, suitably more than 2 apertures, preferably more than 4 apertures, for example 6 or more apertures. Such modern diesel engines may be characterised by an operating tip temperature in excess of250°C. Such modern diesel engines may be characterised by a a fuel injection system which provides a fuel pressure of more than 1350 bar, preferably more than 1500 bar, more preferably more than 2000 bar. Preferably, the diesel engine has fuel injection system which comprises a common rail injection system. The method and use of the present invention preferably improve the performance of an engine having one or more of the above-described characteristics. The method and use of the present invention improve the performance of an engine. This improvement in performance is suitably achieved by reducing deposits in the engine. The present invention may therefore provide a method of combating deposits in an engine comprising combusting in said engine a fuel composition of the third aspect. Combating deposits may involve reducing or the preventing of the formation of deposits in an engine compared to when running the engine using unadditised fuel. Such a method may be regarded as achieving “keep clean” performance. Combating deposits may involve the removal of existing deposits in an engine. This may be regarded as achieving “clean up” performance. In especially preferred embodiments the method and use of the fourth and fifth aspects of the present invention may be used to provide “keep clean” and “clean up” performance. As explained above deposits may occur at different places within a diesel engine, for example a modern diesel engine. The present invention is particularly useful in the prevention or reduction or removal of internal deposits in injectors of engines operating at high pressures and temperatures in which fuel may be recirculated and which comprise a plurality of fine apertures through which the fuel is delivered to the engine. The present invention may also provide improved performance in modern diesel engines having a high pressure fuel system by controlling external injector deposits, for example those occurring in the injector nozzle and / or at the injector tip. The ability to provide control of internal injector deposits and external injector deposits is a useful advantage of the present invention. Suitably the present invention may reduce or prevent the formation of external injector deposits. It may therefore provide “keep clean” performance in relation to external injector deposits. Suitably the present invention may reduce or remove existing external injector deposits. It may therefore provide “clean up” performance in relation to external injector deposits. Suitably the present invention may reduce or prevent the formation of internal diesel injector deposits. It may therefore provide “keep clean” performance in relation to internal diesel injector deposits. Suitably the present invention may reduce or remove existing internal diesel injector deposits. It may therefore provide “clean up” performance in relation to internal diesel injector deposits. The removal or reduction of IDIDs according to the present invention will lead to an improvement in performance of the engine. The improvement in performance of the diesel engine system may be measured by a number of ways. Suitable methods will depend on the type of engine and whether “keep clean” and / or “clean up” performance is measured. An improvement in “keep clean” performance may be measured by comparison with a base fuel. “Clean up” performance can be observed by an improvement in performance of an already fouled engine. The effectiveness of fuel additives is often assessed using a controlled engine test. In Europe the Co-ordinating European Council for the development of performance tests for transportation fuels, lubricants and other fluids (the industry body known as CEC), has developed a test for additives for modern diesel engines such as HSDI engines. The CEC F-98-08 test is used to assess whether diesel fuel is suitable for use in engines meeting new European Union emissions regulations known as the “Euro 5” regulations. The test is based on a Peugeot DW10 engine using Euro 5 injectors, and is commonly referred to as the DW10 test. This test measures power loss in the engine due to deposits on the injectors, and is further described in example 13. Preferably the use of the fuel composition of the present invention leads to reduced deposits in the DW10 test. For “keep clean” performance a reduction in the occurrence of deposits is preferably observed. For “clean up” performance removal of deposits is preferably observed. The DW10 test is used to measure the power loss in modern diesel engines having a high pressure fuel system. Suitably the use of a fuel composition of the present invention may provide a “keep clean” performance in modern diesel engines, that is the formation of deposits on the injectors of these engines may be inhibited or prevented. Preferably this performance is such that a power loss of less than 5%, preferably less than 2% is observed after 32 hours as measured by the DW10 test. In some embodiments, the present invention may provide a power gain. Suitably when combusting a fuel composition according to the present invention a power gain in the DW10 test is observed compared to when combusting an unadditised base fuel and with clean injectors. Suitably a power gain of at least 0.5%, preferably at least 1% is achieved within 4 hours, preferably within 2 hours. Suitably the use of a fuel composition of the present invention may provide a “clean up” performance in modern diesel engines, that is deposits on the injectors of an already fouled engine may be removed. Preferably this performance is such that the power of a fouled engine may be returned to within 1% of the level achieved when using clean injectors within 16 hours, preferably 12 hours, more preferably 8 hours as measured in the DW10 test. Preferably rapid “clean-up” may be achieved in which the power is returned to within 1% of the level observed using clean injectors within 4 hours, preferably within 2 hours. In some preferred embodiments, clean up may also provide a power increase. Thus a fouled engine may be treated to remove the existing deposits and provide an additional power gain. Clean injectors can include new injectors or injectors which have been removed and physically cleaned, for example in an ultrasound bath. The present invention may improve the performance of a diesel engine by combatting internal diesel injector deposits or IDIDs in the injectors of a severely fouled engine. The present invention may clean up internal diesel injector deposits caused by lacquers and / or carboxylate residues. The present invention may clean up internal diesel injector deposits caused by amide lacquers and / or carboxylate residues. The present invention may clean up internal diesel injector deposits caused by lacquers. The present invention may clean up internal diesel injector deposits caused by amide lacquers. Preferably the present invention clean up internal diesel injector deposits caused by carboxylate residues. Carboxylate residues are typically present as metal or ammonium salts. “Clean-up” of a fouled engine may provide significant advantages. For example, superior clean up may lead to an increase in power and / or an increase in fuel economy and / or reduced emissions. In addition removal of deposits from an engine, in particular from injectors may lead to an increase in interval time before injector maintenance or replacement is necessary thus reducing maintenance costs. In some preferred embodiments, clean up may also provide a power increase. Thus a fouled engine may be treated to remove the existing deposits and provide an additional power gain. The removal of IDIDs according to the present invention will lead to an improvement in performance of the engine. The improvement in performance of the diesel engine system may be measured by a number of ways. “Clean up” performance can be observed by an improvement in performance of an already fouled engine. A controlled engine test which has been developed to assess the control of IDIDs is commonly known as the DW10C test. This test assesses the ability of a fuel composition to prevent the formation of IDIDs that lead to injector sticking. The DW10C test procedure was developed by CEC as a “keep clean” procedure test and thus may be used to measure the “keep clean” performance of an engine. However it is often modified and used as a clean up procedure and thus can also be used to measure the “clean up” performance of an engine. The DW10C test is described in example 11. Reference herein to the DW10C test means the test method described in example 11. A modified version of this test adapted to measure clean up, is described in example 12. This modified test was used to test the additives of the invention. In the DW10C test the performance of the engine is rated using a merit score. The maximum score is 10 and a score in excess of 9.5 indicates an exceptional performance. A score of less than 8 indicates that the engine is severely fouled and likely contains very high levels of IDIDs. Very surprisingly additive combinations according to the present invention have been found to perform exceptionally well in the DW10C test when used to clean up a severely fouled engine. By a severely fouled engine we mean to refer to an engine which would achieve a merit rating of less than 8 In the DW10C test. In particular the method and use of the present invention can clean up IDIDs from an engine with a level of fouling equivalent to a rating of less than 8 in the DW10C test. Preferably the use and method according to the present invention provide a score in a DW10C clean up test in excess of 9.8, preferably in excess of 9.9 when introduced to an engine having a merit score of less than 8, following the treatment with an equivalent fuel absent the additive. As is described in example 8, some additive combinations of the present invention may achieve a score of 10 in the DW10C test when used to clean up an engine with a level of fouling to give a merit score of less than 8, for example less than 7.8 or less than 7.6, following treatment with an equivalent fuel absent the additive. The present invention provides improved performance in modern diesel engines having a high pressure fuel system by controlling internal diesel injector deposits and external injector deposits, for example those occurring in the injector nozzle and / or at the injector tip. The ability to provide control of internal injector deposits and external injector deposits is a useful advantage of the present invention. The diesel fuel compositions of the present invention may also provide improved performance when used with traditional diesel engines. Preferably the improved performance is achieved when using the diesel fuel compositions in modern diesel engines having high pressure fuel systems and when using the compositions in traditional diesel engines. This is important because it allows a single fuel to be provided that can be used in new engines and older vehicles. For older engines an improvement in performance may be measured using the XUD9 test. This test is described in relation to example 14. Suitably the use of a fuel composition of the present invention may provide a “keep clean” performance in traditional diesel engines, that is the formation of deposits on the injectors of these engines may be inhibited or prevented. Preferably this performance is such that a flow loss of less than 50%, preferably less than 30% is observed after 10 hours as measured by the XUD-9 test. Suitably the use of a fuel composition of the present invention may provide a “clean up” performance in traditional diesel engines, that is deposits on the injectors of an already fouled engine may be removed. Preferably this performance is such that the flow loss of a fouled engine may be reduced by 10% or more within 10 hours as measured in the XUD-9 test. The benefits provided by the present invention mean that engines need to be serviced less frequently, leading to cost savings and an increase in maintenance intervals. The additives of the present invention may provide a further benefit in addition to those listed above. For example the additive may provide lubricity benefits and / or corrosion inhibition and / or cold flow improvement. In one embodiment the present invention provides the use of at least one quaternary ammonium salt comprising cations Q+ and anions An- as an additive to improve the lubricity of a diesel fuel composition; wherein Q+ is a quaternary ammonium cation; An- is an itaconic acid based anion; and n is at least 1. The present invention may also combat deposits on vehicle fuel filters. This may include reducing or preventing the formation of deposits (“keep clean” performance) or the reduction or removal of existing deposits (“clean up” performance). In some embodiments the method and use of the fourth and fifth aspects of the present invention may improve the performance of a spark ignition engine. In preferred embodiments the spark ignition engine is a direct injection spark ignition engine. The improvement in performance of the gasoline engine system may be measured by a number of ways. Suitably the method and use of the present invention provides in a direct injection spark ignition engine one or more of: improved fuel economy reduced maintenance less frequent overhaul or replacement of injectors improved driveability improved power improved acceleration. In some embodiments the method and use of the fourth and fifth aspects of the present invention combat deposits in a spark ignition engine, preferably a direct injection spark ignition engine. In some embodiments the present invention provides a method and use for removing deposits in a direct injection spark ignition engine. The removal of deposits may be regarded as providing “clean-up” of the engine. “Clean-up” of a fouled engine may provide significant advantages. For example, superior clean up may lead to an increase in power and / or an increase in fuel economy and / or a reduction in pollutant emissions. In addition removal of deposits from an engine, in particular from injectors may lead to an increase in interval time before injector maintenance or replacement is necessary thus reducing maintenance costs. In some preferred embodiments, clean up may also provide a power increase. “Clean up” performance can be observed by an improvement in performance of an already fouled engine. The effectiveness of fuel additives is often assessed using a controlled engine test. A CEC test for injector fouling in DISI engines (TDG-F-113) is in development. A preliminary version of the upcoming test was published by D. Weissenberger, J. Pilbeam, "Characterisation of Gasoline Fuels in a DISI Engine", lecture held at Technische Akademie Esslingen, June, 2017. The test engine is a VW EA111 1 4L TSI engine with 125 kW. The test procedure is a steady state test at an engine speed of 2000 rpm and a constant torque of 56 Nm. This test is available at multiple ISO accredited independent test providers. To measure the clean up performance of an additive, an engine test typically involves a first phase in which unadditised fuel is combusted in an engine. This leads to deposit formation and is regarded as the “dirty up” phase. The fuel is then switched to an additised fuel and the effectiveness of this fuel at cleaning up the engine in the “clean-up” phase is assessed. In a controlled direct injection spark ignition engine test, engine speed and load is held within a tight tolerance. In order to do this the engine control unit adjusts the injection time (or pulse width) to maintain engine performance. In a fouled engine, injection time is increased. Clean up performance of an additised fuel composition can be assessed by measuring the time taken for the injection time to return to the initial injection time when using clean injectors. For the avoidance of doubt, in the context of the direct injection spark ignition engine, by the term “injection time” we mean to refer to the duration of the injection of fuel into the combustion chamber. Preferably the method and the use of the present invention restore the injection time of a fouled engine to within 10% of the initial injection time when using clean injectors in a period of less than 10 hours, preferably less than 8 hours, for example less than 6 hours. Brief Description of the Figures For a better understanding of the invention, and to show how example embodiments may be carried into effect, reference will now be made to the accompanying figures in which: Figure 1 shows the cycle profile used in the test method CEC F-110-16 according to Example 12. Figure 2 shows an example of all exhaust temperatures with <30°C deviation, indicating no sticking caused by IDID according to Example 12. The invention will now be further described with reference to the following non-limiting examples. In the examples which follow the values given in parts per million (ppm) for treat rates denote active agent amount, not the amount of a formulation as added, and containing an active agent. All parts per million are by weight. Example 1 A partial ester of itaconic acid was prepared as follows: To a 1L reactor charged with 2-ethylhexanol (250g, 1.918 moles) was added toluene (215.7g) and heated to 90°C. To the stirred liquid was added itaconic acid (250g, 1.921 moles) and p-toluenesulfonic acid (3.31g). The reaction was heated towards 120°C, whilst removing water by distillation over 7 hours. The products were cooled to room temperature and unreacted itaconic and p-toluenesulfonic acid removed by filtration and washing with water. The toluene was removed on a rotary evaporator to leave a yellow / orange liquid (2-ethylhexyl itaconate, 412.9g). Example 2 Poly (2-ethylhexyl itaconate) was prepared as follows: 2-ethylhexyl itaconate (example 1, 400.05g) was heated to 70°C under nitrogen and tert-Butyl peroxypivalate (6 x 2.4ml, Trigonox 25-c75) was added over 5 hours, followed by mixing at 70°C for 2 hours. Aromatic 150 (160.54g) was added to leave a golden yellow, clear viscous liquid (560.50g). Example 3 Additive A, a quaternary ammonium compound of the present invention was prepared as follows: Poly (2-ethylhexyl itaconate) (607.18g, example 2) was charged to a 1 litre oil jacketed reactor, mixed with 2-ethylhexanol (147.39g) and heated to 50°C. Dimethylaminoethanol (52.55g) was added slowly over 15 minutes. 1,2-epoxybutane (42.6g) was added and the reaction mixed at 60°C for 13 hours to leave a yellow amber, clear viscous liquid (824.53g)). Example 4 Poly (2-ethylhexyl itaconate) was prepared as follows: 2-ethylhexyl itaconate (800.63g, 50% in 2-ethylhexanol) was charged to a 1 litre oil jacketed reactor and heat to 70°C, before adding tert-butyl peroxypivalate (6 x 4.8ml, Trigonox 25-c75) over 5 hours. 2-ethylhexanol (180.02g) was added and additional tert-butyl peroxypivalate (4.8ml, Trigonox 25-c75), mixed for 2 hours at 70°C. Another charge of tert-butyl peroxypivalate (4.8ml, Trigonox25-c75) was added and mixed fora further2 hours, to leave a yellow / amber, clear viscous liquid (975.98g). Example 5 Additive B, a quaternary ammonium compound of the present invention was prepared as follows: Poly (2-ethylhexyl itaconate) (800.76g, 40% in 2-Ethylhexanol, example 4) was charged to a 1 litre oil jacketed reactor and mixed at 50°C. Dimethylaminoethanol (116.245g) was added slowly over 30 minutes. 1,2-epoxybutane (94.1g) was added and the reaction mixed at 60°C for 8 hours. The product was diluted with 2-ethylhexanol (150.09g) and Aromatic 150 (282.44g) to leave a clear, amber yellow viscous liquid (1361.93g). A quaternary ammonium compound not of formula (X) was prepared as follows: N-oleyldiethanolamine (237.89g) and 2-ethylhexanol (60g) were charged to a 1 litre pressure reactor and mixed at ambient temperature. Dimethylcarbonate (73.37g) was added and the reaction heated to 80-100°C for 2 hours (3 bar), then 130°C for 24 hours (2.5 bar). The volatiles were removed on a rotary evaporator to leave an amber / brown clear liquid (306.59g). Example 7 Additive C, a quaternary ammonium compound of the present invention was prepared by an anion exchange reaction, as follows: Bis(2-hydroxyethyl) methyl oleyl ammonium methylcarbonate (70.01g, example 6) was charged to a 500ml RB flask and heated to 50°C. Poly (2-ethylhexyl Itaconate) (183.62g, example 4) was added and when the exotherm subsided, the reaction was heated to 120-130°C for 2 hours, then to 140°C for 2 hours to leave an amber, clear viscous liquid (239.8g). Example 8 Additive D, a quaternary ammonium compound of the present invention was prepared by an anion exchange reaction, as follows: Bis(2-hydroxyethyl) dimethyl oleyl ammonium methylcarbonate (90.25g, example 6) was charged to a 500ml RB flask and heated to 50°C. Poly (2-ethylhexyl Itaconate) (118.32g, example 4) was added and when the exotherm subsided, the reaction was heated to 120-140°C for 15 hours to leave a clear amber viscous liquid (181,17g). Example 9 Additive E, a quaternary ammonium salt was prepared according to the procedure set out in examples 1 and 3 of WO2023 / 111550. Example 10 Additive F, a quaternary ammonium compound of the present invention was prepared as follows: Poly (2-ethylhexyl itaconate) (159.1g, 40% in 2-Ethylhexanol, example 4) was charged to a 500ml 3 necked round bottom flask and heated to 50°C with stirring. Dimethyloctadecylamine (75.38g) was added over 10 minutes. 2-ethylhexyl glycidyl ether (47.05g) was added and the reaction stirred at 60°C for 8 hours. The product was diluted with Aromatic 150 (55.5g) to leave a clear, amber yellow viscous liquid (323.8g). 5 Example 11 Fuel compositions were prepared by adding additives A to F to diesel fuel in the amounts set out in table 1. The amounts specified are parts per million (ppm) by weight of the active additive component, ignoring any diluent or carrier. 10 Table 1 Fuel Composition Additive A (PPm active) Additive B (PPm active) Additive D (PPm active) Additive E (PPm active) Additive F (PPm active) 1 40 45 2 40 45 3 40 45 4 48 45 5 25 60 6 25 60 7 60 68 8 27.5 60 9 40 45 10 27.5 60 11 40 45 12 60 The diesel fuel complied with the RF06 base fuel, the details of which are given in table 2 15 below. Table 2 Property Units Limits Min Max Method Cetane Number 52.0 54.0 EN ISO 5165 Density at 15°C kg / m3 833 837 EN ISO 3675 Distillation 50% v / v Point 95% v / v Point °C °C 245 345 350 FBP °C - 370 Flash Point °C 55 - EN 22719 Cold Filter Plugging °C - -5 EN 116 Point Viscosity at 40°C mm2 / sec 2.3 3.3 EN ISO 3104 Polycyclic Aromatic % m / m 3.0 6.0 IP 391 Hydrocarbons Sulphur Content mg / kg - 10 ASTM D 5453 Copper Corrosion - 1 EN ISO 2160 Conradson Carbon Residue on % m / m - 0.2 EN ISO 10370 10% Dist. Residue Ash Content % m / m - 0.01 EN ISO 6245 Water Content % m / m - 0.02 EN ISO 12937 Neutralisation (Strong Acid) mg KOH / g - 0.02 ASTM D 974 Number Oxidation Stability mg / mL - 0.025 EN ISO 12205 HFRR (WSD1,4) pm - 400 CEC F-06-A-96 Fatty Acid Methyl Ester prohibited Fuel composition 13 was prepared by dosing 40 ppm active additive A and 45 ppm active E into a B10 fuel comprising 90 vol% of an RF-79-07 diesel mineral fuel complying with the above specification and 10 vol% rapeseed methyl esters (RME). 5 Example 12 The ability of additives of the invention to remove ‘Internal Diesel Injector Deposits’ (IDIDs) may be measured according to the test method CEC F-110-16, available from the Co-10 ordinating European Council. The test uses the PSA DW10C engine. The engine characteristics as follows: Design: Four cylinders in line, overhead camshaft, variable geometry tuibochargei with EGR Capacity; 1997 cmJ Combustion chamber: Four valves, bowl m piston, direct injection Power: 120 kW 3750 rpm Torque: 340 Nm @ 2000 rpm Injection system: Common rail with solenoid type injectors Delphi Injection System Emissions control: Conforms to Euro V limit values when combined with exhaust gas post-treatment system. The test fuel (RF06) is dosed with 0.5mg / kg Na in the form of Sodium Naphthenate + 10mg / kg Dodecyl Succinic Acid (DDSA). The test procedure consists of main run cycles followed by soak periods, before cold starts are carried out. The main running cycle consist of two speed and load set points, repeated for 6hrs, as seen below. Step Speed (rpm) Torque (N.m) Duration (s) 1 3750 280 1470 1 - Ramp - - 3D •7 1000 10 270 2 - Ramp 1 - - 30 The ramp times of 30 seconds are included m the duration of each step. The cycle profile is shown in Figure 1. Each cycle is repeated 6 times so the complete main run phase lasts 6 hours. During the main run, parameters including, Throttle pedal position, ECU fault codes, Injector balance coefficient and Engine stalls are observed and recorded. The engine is then left to soak at ambient temperature for 8hrs. After the soak period the engine is re-started. The starter is operated for 5 seconds; if the engine fails to start the engine is left for 60 seconds before a further attempt. A maximum of 5 attempts are allowed. If the engine starts the engine is allowed to idle for 5 minutes. Individual exhaust temperatures are monitored and the maximum Temperature Delta is recorded. An increased variation in Cylinder-to-Cylinder exhaust temperatures is a good indication that injectors are suffering from IDID. Causing them to either open slowly or stay open to long. An example of all exhaust temperatures with <30°C deviation, indicating no sticking caused by IDID is shown in Figure 2. The complete test comprises of 6x Cold Starts, although the Zero hour Cold Start does not form part of the Merit Rating and 5x6hr Main run cycles, giving a total of 30hrs engine running time. The recorded data is inputted into the Merit Rating Chart. This allows a Rating to be produced for the test. Maximum rating of 10 shows no issues with the running or operability of the engine for the duration of the test. An example below: 5 Cold start Starting Exhaust temperature consistency Cold start Start Y / N Maximus Merits Number of attempts (1 = first start) Deduction Merits— Maximums Merits Exhaust temperature Max cyl. Deviation (°C) Deduction #0 not rated #1 Y 5 1 0 5 5 21.8 0 #2 Y 5 1 0 ..................................... 5 18.1 0 #3 Y 5 1 0 5 5 15.5 0 #4 Y 5 1 0 5 5 20.2 0 #5 Y 5 1 0 5 5 22.6 0 Total merits 25 25 Main run Operability Main run Maximum Merits Number of ECU Fault resets Deduction Stall (Y / N) Deduction Max pedal position at 1000 rpm / 10 N.m (%) Deduction Max inject. Balancing Coeff. (rpm) Deduction #1 5 0 0 N 0 15.4 0 15 0 5 #2 5 0 0 N 0 13.5 0 15 0 5 #3 5 0 0 N 0 13.6 0 16 0 5 #4 5 0 0 N 0 13.8 0 15 0 5 #5 5 0 0 N 0 14.5 0 15 0 5 25 Global Rating - Summary (Merit / 10) 10 The propensity of the test fuel to cause injector deposits (IDID) is evaluated through the following criteria: □ Cold start parameters: 1. Number of failed starts. 2. Exhaust temperature deviation from standard value for cylinders 1 to 4 □ Main run parameters 1. Number of engine stalls 2. Number of IDID related ECU faults generated during main run 3. Pedal position drift on low speed phases 4. Injector balancing Note: 1st Cold start (#0) is run with Flush fuel and is not rated The rating can be summarized as follows: 1 / Cold Start (for start #1 to #5): Startability rating: □ 1st start: merit = 5 / each fail brings a “-1” merit discount. Maximum Exhaust Ports Temperature deviation rating: □ merit = 5 if T<30°C I 2 if 30°C<T<50°C 10 if T>50°C. Cold Start Rating range: 0 -> 10 for each Cold Start (5 Cold Starts rated in total) 2 / Main Run (for run #1 to #5): Operability rating: □ merit = 5 if no stall and no IDID related ECU Fault, each IDID related ECU fault brings a “-1” merit discount (after 5th ECU Fault Reset -> Next cold start). □ merit = 0 if stall (Then -> Next Cold Start). Maximum Pedal Position: □ merit = 5 if P<25% I 2 if 25%<P<40% 10 if P>40% Maximum Injector Balancing Factror deduction: □ merit = 5 if IB<20rpm I 2 if 30rpm<IB<20rpm 10 if IB>30rpm Main Run Rating range: 0 -> 5 for each Main Run (5 in total) Maximum global rating value: 75 (ie: 5x 10 + 5x5). Global rating = 10 x (Cold Start + Main Run Rating values) / 75 Resulting in 0 to 10 merit scale The ability of additives of the invention to clean up IDIDs may be assessed according to a modification of the DW10C test described in example 12. The In-House Clean-Up Method developed starts by running the engine using reference diesel (RF06) dosed with 0.5mg / kg Na + 10mg / Kg DDSA until an exhaust temperature Delta of >50°C is observed on the Cold Start. This has repeatedly been seen on the 3rd Cold Start which follows the second main run, 12hrs total engine run time. Once the increased Exhaust temperature Delta is observed, the engine fuel supply is swapped to reference diesel, dosed with 0.5mg / kg Na (as sodium naphthenate) + 10mg / kg DDSA + the Candidate sample. The fuel is flushed through to the engine and allowed to commence with the next Main run. The ability of the Candidate additive to prevent any further increase in deposits or to remove the deposits can then be determined as the test continues. Example 14 The performance of fuel compositions of the invention in modern diesel engines having a high pressure fuel system may be tested according to the CECF-98-08 DW 10 method. This is referred to herein as the DW10B test. The engine of the injector fouling test is the PSA DW10BTED4. In summary, the engine characteristics are: Design: Four cylinders in line, overhead camshaft, turbocharged with EGR Capacity: 1998 cm3 Combustion chamber: Four valves, bowl in piston, wall guided direct injection Power: 100 kW at 4000 rpm Torque: 320 Nm at 2000 rpm Injection system: Common rail with piezo electronically controlled 6-hole injectors. Max. pressure: 1600 bar (1.6 x 108 Pa). Proprietary design by SIEMENS VDO Emissions control: Conforms with Euro IV limit values when combined with exhaust gas posttreatment system (DPF) This engine was chosen as a design representative of the modern European high-speed direct injection diesel engine capable of conforming to present and future European emissions requirements. The common rail injection system uses a highly efficient nozzle design with rounded inlet edges and conical spray holes for optimal hydraulic flow. This type of nozzle, when combined with high fuel pressure has allowed advances to be achieved in combustion efficiency, reduced noise and reduced fuel consumption, but are sensitive to influences that can disturb the fuel flow, such as deposit formation in the spray holes. The presence of these deposits causes a significant loss of engine power and increased raw emissions. The test is run with a future injector design representative of anticipated Euro V injector technology. It is considered necessary to establish a reliable baseline of injector condition before beginning fouling tests, so a sixteen hour running-in schedule for the test injectors is specified, using non-fouling reference fuel. Full details of the CEC F-98-08 test method can be obtained from the CEC. The coking cycle is summarised below. 1. A warm up cycle (12 minutes) according to the following regime: Step Duration (minutes) Engine Speed (rpm) Torque (Nm) 1 2 idle <5 2 3 2000 50 3 4 3500 75 4 3 4000 100 2. 8 hrs of engine operation consisting of 8 repeats of the following cycle Step Duration (minutes) Engine Speed (rpm) Load (%) Torque (Nm) Boost Air After IC (°C) 1 2 1750 (20) 62 45 2 7 3000 (60) 173 50 3 2 1750 (20) 62 45 4 7 3500 (80) 212 50 5 2 1750 (20) 62 45 6 10 4000 100 * 50 7 2 1250 (10) 20 43 8 7 3000 100 * 50 9 2 1250 (10) 20 43 10 10 2000 100 * 50 11 2 1250 (10) 20 43 12 7 4000 100 * 50 for expected range see CEC method CEC-F-98-08 3. Cool down to idle in 60 seconds and idle for 10 seconds 4. 4 hrs soak period The standard CEC F-98-08 test method consists of 32 hours engine operation corresponding to 4 repeats of steps 1-3 above, and 3 repeats of step 4. ie 56 hours total test time excluding warm ups and cool downs. Example 15 The effectiveness of the additives of the invention in older traditional diesel engine types may be assessed using a standard industry test - CEC test method No. CEC F-23-A-01. This test measures injector nozzle coking using a Peugeot XUD9 A / L Engine and provides a means of discriminating between fuels of different injector nozzle coking propensity. Nozzle coking is the result of carbon deposits forming between the injector needle and the needle seat. Deposition of the carbon deposit is due to exposure of the injector needle and seat to combustion gases, potentially causing undesirable variations in engine performance. The Peugeot XUD9 A / L engine is a 4 cylinder indirect injection Diesel engine of 1.9 litre swept volume, obtained from Peugeot Citroen Motors specifically for the CEC PF023 method. The test engine is fitted with cleaned injectors utilising unflatted injector needles. The airflow at various needle lift positions have been measured on a flow rig prior to test. The engine is operated for a period of 10 hours under cyclic conditions. Stage Time (secs) Speed (rpm) Torque (Nm) 1 30 1200 ±30 10±2 2 60 3000 ± 30 50 ±2 3 60 1300 ±30 35 ±2 4 120 1850 ±30 50 ±2 The propensity of the fuel to promote deposit formation on the fuel injectors is determined by measuring the injector nozzle airflow again at the end of test, and comparing these values to those before test. The results are expressed in terms of percentage airflow reduction at various needle lift positions for all nozzles. The average value of the airflow reduction at 0.1 mm needle lift of all four nozzles is deemed the level of injector coking for a given fuel. In a clean up test a first test cycle (dirty-up phase) is carried out using an unadditised fuel. A further test is then carried out starting with the dirty injectors but using additised fuel. The percentage clean up is recorded as the relative improvement in flow loss compared with the flow loss at the end of the dirty up phase. Example 16 Fuel compositions 1 to 5 were tested for their ability to clean up deposits in traditional diesel engines following the procedure set out in example 15. The results are shown in table 3: Table 3 Fuel Composition Flow loss after dirty up (%) Flow loss after clean up (%) % clean up 1 71.66 13.22 82 2 71.91 21.72 70 3 73.10 1.40 98 4 67.13 2.59 96 5 68.95 3.41 95 Example 16 Diesel fuel compositions 6 and 7 were tested according to the CEC F-98-08 DW10B test method described in example 14, modified to measure clean up performance as outlined below. A first 32 hour cycle was run using new injectors and RF-06 base fuel having added thereto 1ppm Zn (as neodecanoate). This resulted in a level of power loss of due to fouling of the injectors. A second 32 hour cycle was then run as a ‘clean up’ phase. The dirty injectors from the first phase were kept in the engine and the fuel changed to RF-06 base fuel having added thereto 1ppm Zn (as neodecanoate) and the test additives. The power output at the end of this cycle was also recorded, compared with the power at the start of the test. The results are shown in table4: Table 4 Fuel Composition Power after dirty up (%) Power after clean up (%) 6 -5.55 -1.05 7 -7.70 -0.21 Example 17 Fuel composition 8 was tested according to the procedure set out in example 12. A merit rating of 10 was achieved. Example 18 Fuel compositions 9 to 11 and 13 were tested according to the procedure set out in example 13. For composition 11 no sodium naphthenate or DDSA was added during the clean up cycle. The results are shown in table 5. Table 5 Fuel Composition Dirty up merit rating Clean up merit rating 9 8 10 10 7.7 9.9 11 6.53 9.6 13 8.9 9.9 Example 19 Additive F, a quaternary ammonium compound of the present invention was prepared as follows: Poly (2-ethylhexyl itaconate) (159.1g, 40% in 2-Ethylhexanol, example 4) was charged to a 500ml 3 necked round bottom flask and heated to 50°C with stirring. Dimethyloctadecylamine (75.38g) was added over 10 minutes. 2-ethylhexyl glycidyl ether (47.05g) was added and the reaction stirred at 60°C for 8 hours. The product was diluted with Aromatic 150 (55.5g) to leave a clear, amber yellow viscous liquid (323.8g). Example 20 Composition 12 was tested according to the CEC test method No. CEC F-23-A-01 set out in example 15. A single cycle was run as a keep clean test. A flow loss of 0.6 % was observed.

Claims

1. An additive composition for a fuel or lubricating oil comprising at least one quaternary ammonium salt comprising cations Q+ and anions An-; wherein Q+ is a quaternary ammonium cation; An- is an itaconic acid based anion; and n is at least 1.

2. An additive composition according to claim 1 which further comprises a diluent or carrier.

3. A lubricating composition comprising an oil of lubricating viscosity and as an additive at least one quaternary salt comprising cations Q+ and anions An-; wherein Q+ is a quaternary ammonium cation; An- is an itaconic acid based anion; and n is at least 1.

4. A fuel composition comprising as an additive at least one quaternary ammonium salt comprising cations Q+ and anions An-; wherein Q+ is a quaternary ammonium cation; Ad- is an itaconic acid based anion; is at least 1.

5. A method of improving the performance of an engine, the method comprising combusting in said engine a fuel composition comprising as an additive at least one quaternary ammonium salt comprising cations Q+ and anions An-; wherein Q+ is a quaternary ammonium cation; An- is an itaconic acid based anion; and n is at least 1.

6. The use of at least one additive in a fuel composition to improve the performance of an engine combusting said fuel composition wherein the or each additive is a quaternary ammonium salt comprising cations Q+ and anions An- wherein Q+ is a quaternary ammonium cation; An- is an itaconic acid based anion; and n is at least 1.

7. A composition, use or method according to any preceding claim wherein the quaternary ammonium salt comprises cations Q+ which are the reaction of:(a) a compound including a tertiary amino group; and(b) epoxides, esters of a carboxylic acid and hydrocarbyl carbonates; and anions An-derived from(c) itaconic acid or an ester and / or polymer thereof.

8. A composition, method or use according to claim 7 wherein the compound including a tertiary amino group (a) is selected from:(i) the reaction product of a hydrocarbyl-substituted acylating agent and a compound having at least one tertiary amine group and a primary amine, secondary amine or alcohol group;(ii) a Mannich reaction product comprising a tertiary amine group;(iii) a polyalkylene substituted amine having at least one tertiary amino group;(iv) a tertiary amine of formula R5R6R7N, wherein each of R5, R6 and R7 isindependently an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group;(v) a cyclic tertiary amine; and(vi) a polyetheramine compound.

9. A composition, method or use according to claim 8 wherein the compound including a tertiary amino group (a) comprises the reaction product of a hydrocarbyl-substituted succinic acid derivative and an amine of formula (7) or (8):R2 R2N--X--NHR4 / N--X---[O(CR42)m]nOHR3 R3(7) (8)wherein R2 and R3 are the same or different alkyl groups having from 1 to 36 carbon atoms; X is an alkylene group having from 1 to 20 carbon atoms and wherein the alkylene group may be optionally interrupted with one or more heteroatoms; n is from 0 to 20; m is from 1 to 5; and R4 is hydrogen or a Ci to C36 alkyl group.

10. A composition, method or use according to claim 8 or claim 9 wherein the compound including a tertiary amino group (a) comprises a compound prepared from the reaction product of a hydrocarbyl substituted succinic acid or anhydride thereof, and an alcohol or amine selected from dimethylaminopropanol, dimethylaminopropylamine, N,N-diethyl-1,3- diaminopropane, N,N-dimethylethylenediamine, N,N-diethylethylenediamine, N,N-dibutylethylenediamine, 3-(2-(dimethylamino)ethoxy) propylamine or combinations thereof.

11. A composition, method or use according to claim 7 or claim 8 wherein the compound including a tertiary amino group (a) is (iv) a tertiary amine of formula R5R6R7N, wherein each of R5, R6 and R7 is independently an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group.

12. A composition, method or use according to claim 11 wherein each of R5, R6 and R7 is independently selected from an alkyl or hydroxyalkyl group having 1 to 10, preferably 1 to 6 carbon atoms, for example 1 to 4 carbon atoms.

13. A composition, method or use according to claim 11 wherein one or two of the groups R5, R6 and R7 is a short chain alkyl or hydroxyalkyl group having 1 to 6, preferably 1 to 4 carbon atoms and the other one or two groups is a longer chain alkyl or alkenyl group having 6 to 30, preferably 10 to 24 carbon atoms.

14. A composition, method or use according to any of claims 11, 12 or 13 wherein component (a) is selected from N,N-dimethyl ethanolamine, dimethyl oleylamine, N-oleyldiethanolamine, dimethyloctadecylamine, dimethyloctadecylamine, N-oleyl diethanolamine, hexadecyl dimethyl amine, N-oleyl diethanolamine and N-methyl-N,N-ditallowamine.

15. A composition, method or use according to any of claims 7 to 14 wherein component (b) is selected from ethylene oxide, propylene oxide, butylene oxide, pentylene oxide, hexylene oxide, heptylene oxide, dodecylene oxide, alkyl glycidyl ethers, for example 2-ethylhexyl glycidyl ether or isopropyl glycidyl ether, alkyl glycidyl esters styrene oxide, stilbene oxide and other C2 to C30 hydrocarbyl groups.

16. A composition, method or use according to any of claims 7 to 14 wherein component (b) is selected from dimethyl carbonate and diethyl carbonate.

17. A composition method or use according to any of claims 7 to 14 wherein component 9b) is selected from dimethyl oxalate and methyl salicylate.

18. A composition, method or use according to any of claims 7 to 17 wherein component (c) comprises a polymer of formula (21):wherein z is at least 2; x + y = 1; and each R is independently hydrogen or an optionally substituted hydrocarbyl group provided that, in the polymer as a whole, not all of the R groups are a hydrocarbyl group.

19. A composition, method or use according to any of claims 7 to 18 wherein component (c) comprises the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is a (preferably branched) alkyl group having 6 to 30, preferably 6 to 24, carbon atoms., or wherein component (c) comprises the polymerised reaction product of itaconic acid / anhydride and an alcohol of formula R21OH wherein R21 is a (preferably branched) alkyl group having 4 to 10 carbon atoms.

20. A composition, method or use according to any of claims 7 to 19 wherein the fuel is a gasoline fuel.

21. A composition, method or use according to any of claims 7 to 19 wherein the fuel is a diesel fuel.

22. A composition, method or use according to any preceding claim wherein the fuel or additive composition further comprises one or more further detergents selected from:(I) a quaternary ammonium salt additive which is not a quaternary ammonium compound as defined in relation to the first aspect;(II) the product of a Mannich reaction between an aldehyde, an amine and an optionally substituted phenol;(III) the reaction product of a carboxylic acid-derived acylating agent and an amine;(IV) the reaction product of a hydrocarbyl-substituted dicarboxylic acid or anhydride and an amine compound or salt which product comprises at least one amino triazole group; and(V) partial esters of substituted succinic acids.

23. A composition, method or use according to claim 22 wherein the one or more further detergents comprises a quaternary ammonium salt additive (la) which is the quaternised reaction product of a hydrocarbyl substituted succinic acid derived acylating agent and a compound able to react with said acylating agent and which includes a tertiary amine group; wherein each molecule of the hydrocarbyl substituted succinic acid derived acylating agent includes on average at least 1.2 succinic acid moieties.

24. A composition, method or use according to claim 22 or claim 23 the one or more further detergents comprises a quaternary ammonium salt which is the reaction product of:(x) a polyisobutenyl substituted succinic acid or anhydride thereof having a PIB molecular weight of 170 to 2800, preferably 450 to 1500 and including an average of at least 1.2 succinic acid moieties per molecule, and an amine or alcohol selected from dimethylaminopropanol and dimethylaminopropylamine; and(y) a quaternising agent selected from dimethyl oxalate; methyl salicylate; and an epoxide selected from styrene oxide, propylene oxide and butylene oxide, in combination with an acid; preferably wherein the quaternising agent is dimethyl oxalate or methyl salicylate.

25. A method or use according to any of claims 5 to 24 which combats deposits in the engine.

26. A method or use according to any of claims 5 to 25 wherein the engine is a diesel engine, preferably a modern diesel engine having a high pressure fuel system.

27. A method or use according to claim 26 wherein the improvement in performance is selected from one or more of:a reduction in power loss of the engine;a reduction in external diesel injector deposits;a reduction in internal diesel injector deposits;an improvement in fuel economy;a reduction in fuel filter deposits;a reduction in emissions; andan increase in maintenance intervals.

28. A method or use according to any of claims 5 to 27 which combats internal diesel injector deposits.

29. A method or use according to any of claims 5 to 25 wherein the engine is a gasoline engine, preferably a direct injection spark ignition engine.

30. A method or use according to claim 29 wherein the improvement in performance is selected from one or more of:improved fuel economyreduced maintenanceless frequent overhaul or replacement of injectorsimproved driveabilityimproved powerimproved acceleration31. A method or use according to any of claims 5 to 30 which achieves “keep clean” performance.

32. A method or use according to any of claims 5 to 31 which achieves “clean up” performance.

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