Method and use

Abstract

Use of one or more ester compounds as an additive in a diesel fuel composition to reduce the impact of deposits in the exhaust gas recirculation system and / or the post combustion system of a diesel en

Description

The present invention relates to methods and uses for improving the performance of diesel engines. In particular the invention relates to reducing the impact of deposits in the exhaust gas recirculation system and / or the post combustion system of diesel engines, especially modern diesel engines having a high-pressure fuel system. Specifically, the invention relates to ester fuel additives for such methods and uses. The addition of detergent additives to combat deposits in the combustion system of diesel engines, for example in the fuel injection system, is well known and a wide variety of detergents have been developed for this purpose. However less work has been carried out to combat deposits in the exhaust gas recirculation system or the post combustion system. Nevertheless, the presence of deposits in the exhaust gas recirculation system or the post combustion system of a diesel engine can have a significant deleterious effect on the performance of diesel engines, especially modern diesel engines having a high pressure fuel system. Exhaust gas recirculation (EGR) systems are fitted to diesel vehicles to reduce NOx emissions. This is achieved by recirculating exhaust gases to the combustion chamber in a controlled manner and thereby increasing the heat capacity of and reducing the oxygen concentration in gases within the combustion chamber. Several types of EGR systems have been developed. High pressure EGR systems are arranged to divert exhaust gases from the combustion chamber, before the exhaust gases reach any turbocharger and / or diesel particulate filter present in the engine, and supply said exhaust gases to the intake manifold downstream of the compressor. The high pressure EGR system therefore operates on the high pressure sides of the intake and exhaust manifolds and supplies the combustion chamber with unfiltered recirculated exhaust gases. Low pressure EGR systems are arranged to divert exhaust gases from the combustion chamber downstream of any turbocharger and / or diesel particulate filter present in the engine, and to supply said exhaust gases to the intake tract upstream of the compressor. The low pressure EGR system therefore operates on the low pressure sides of the intake and exhaust manifolds and supplies the combustion chamber with filtered recirculated exhaust gases. Hybrid (or combined) EGR systems integrate both high pressure EGR and low pressure EGR on the same engine, to combine the benefits of each system. Dedicated EGR (D-EGR) systems are arranged to route the entire exhaust of a sub-group of power cylinders (dedicated cylinders) directly into the intake manifold. Overtime deposits can form within an EGR system. This is a particular issue in diesel engines with high pressure EGR systems, due to the recirculated exhaust gas stream being taken upstream of any turbocharger and diesel particulate filter, meaning that problematic particulate combustion products are re-introduced into the intake manifold and the combustion chamber. One area where such deposits cause a particular problem is within the cooler component of the EGR system. If the level of deposits becomes significant then the engine management systems in sophisticated diesel engines may cause the engine to operate with reduced performance and / or enter into a safe running mode. This scenario would have significant impact on the vehicle's operability and would require inspection by a suitably qualified workshop. A typical EGR system comprises an intake pipe, a valve, a housing, a cooler and an outlet pipe. Deposits build up on the interior surfaces of all portions of the EGR system, but particularly in the cooler. It would be beneficial to combat such deposits, particularly in the cooler of an EGR system, particularly in the cooler of a high pressure EGR system. The post combustion system of a diesel engine typically includes a series of components through which exhaust gases must flow before exiting the vehicle. The post combustion system may include a turbocharger, a diesel oxidation catalyst, a diesel particulate filter, a selective catalytic reduction unit and an ammonia oxidation catalyst. It would be desirable to combat deposits in any or all of these components. The formation of deposits in the post combustion system may involve the accumulation of soot on components of the post combustion system. In particular, the formation of deposits in the post combustion system may involve an accumulation and / or capture of soot in a diesel particulate filter of the post combustion system. It would also be beneficial to prevent and / or to remove deposits on sensors within the post combustion system, for example deposits on NOx sensors, temperature sensors and / or pressure sensors. It would also be beneficial to prevent and / or to remove the accumulation of soot on components of the post combustion system and / or to prevent and / or to remove soot from a diesel particulate filter of the post combustion system. The present inventors have surprisingly found that the inclusion of certain compounds as fuel additives is able to combat the effect of deposits in the EGR system and / or the post combustion system. According to a first aspect of the present invention there is provided the use of one or more ester compounds as an additive in a diesel fuel composition to reduce the impact of deposits in the exhaust gas recirculation system and / or the post combustion system of a diesel engine when combusting said diesel fuel composition; wherein the one or more ester compounds are the reaction product of: a) an optionally substituted polycarboxylic acid or an anhydride thereof; and b) an alcohol of formula H-(OR)n-OR1, wherein R is an optionally substituted alkylene group, R1 is hydrogen or an optionally substituted hydrocarbyl group and n is 0 or a positive integer; provided that n is not 0 when R1 is hydrogen. According to a second aspect of the present invention there is provided a method of reducing the impact of deposits in the exhaust gas recirculation system and / or the post combustion system of a diesel engine, the method comprising combusting in the engine a diesel fuel composition comprising as an additive one or more ester compounds; wherein the one or more ester compounds are the reaction product of: a) an optionally substituted polycarboxylic acid or an anhydride thereof; and b) an alcohol of formula H-(OR)n-OR1, wherein R is an optionally substituted alkylene group, R1 is hydrogen or an optionally substituted hydrocarbyl group and n is 0 or a positive integer; provided that n is not 0 when R1 is hydrogen. According to a third aspect of the present invention there is provided a diesel fuel composition comprising as an additive one or more ester compounds; wherein the one or more ester compounds are the reaction product of: a) an optionally substituted polycarboxylic acid or an anhydride thereof; and b) an alcohol of formula H-(OR)n-OR1, wherein R is an optionally substituted alkylene group, R1 is hydrogen or an optionally substituted hydrocarbyl group and n is 0 or a positive integer; provided that n is not 0 when R1 is hydrogen; wherein combustion of the diesel fuel composition in a diesel engine reduces the impact of deposits in the exhaust gas recirculation system and / or the post combustion system of a diesel engine compared to when an otherwise identical diesel fuel without the additive is combusted under identical conditions. The present invention relates to a method, use and diesel fuel composition which reduces the impact of deposits in the EGR system and / or the post combustion system of a diesel engine. The presence of deposits on one or more parts of the EGR system and / or the post combustion system of a diesel engine typically has a negative effect on the performance of the engine. Reducing the impact of deposits may involve reducing or preventing the formation of deposits and / or removing existing deposits and / or changing the nature of the deposits. In some embodiments reducing the impact of deposits may involve changing the nature of deposits. This means that the structure or composition of deposits which are formed is different in a way that is less detrimental to the performance of the engine, for example by increasing the combustibility and / or thermal conductivity of the deposits. In some preferred embodiments reducing the impact of deposits involves reducing and / or preventing the formation of deposits and / or the removal of existing deposits. In some embodiments, the first aspect of the present invention provides the use of one or more ester compounds as defined herein as an additive in a diesel fuel composition to reduce the impact of deposits in the exhaust gas recirculation system of a diesel engine when combusting said diesel fuel composition. In some preferred embodiments, the use reduces the formation of deposits in the EGR system. In some embodiments, the second aspect of the present invention provides a method of reducing the impact of deposits in the exhaust gas recirculation system of a diesel engine, the method comprising combusting in the engine a diesel fuel composition comprising as an additive one or more ester compounds as defined herein. In some preferred embodiments, the method reduces the formation of deposits in the EGR system. In some embodiments, the first aspect of the present invention provides the use of one or more ester compounds as defined herein as an additive in a diesel fuel composition to reduce the impact of deposits in the post combustion system of a diesel engine when combusting said diesel fuel composition. In some embodiments, the second aspect of the present invention provides a method of reducing the formation of deposits in the post combustion system of a diesel engine, the method comprising combusting in the engine a diesel fuel composition comprising as an additive one or more ester compounds as defined herein. In preferred embodiments, reducing the impact of deposits involves reducing and / or preventing the impact of deposits in the post combustion system of a diesel engine. The present invention relates to the use of one or more ester compounds as an additive in a diesel fuel composition to reduce the impact of deposits in the EGR system and / or the post combustion system. The present invention involves the use of one or more ester compounds as an additive. By this we mean that the invention may include the use of an ester compound as an additive or the use of multiple ester compounds as multiple additives. For the avoidance of doubt each additive used in the present invention may comprise a mixture of compounds and references to an additive or the additive include mixtures, unless otherwise stated. In particular mixtures of isomers and mixtures of homologues are within the scope of the invention. The skilled person will appreciate that commercial sources of some of the additive compounds and / or reactants used to form the additives described herein may comprise mixtures of isomers and / or mixtures of homologues. For example component a) may be a mixture of optionally substituted polycarboxylic acids or anhydrides thereof, and component b) may be a mixture of alcohols of formula H-(OR)n-OR1. Preferred features of the first and second aspects of the invention will now be described. Any feature of any aspect may be combined with any feature of any other aspect as appropriate. The present invention relates to the use of one or more ester compounds which are the reaction product of an optionally substituted polycarboxylic acid or an anhydride thereof and an alcohol of formula H-(OR)n-OR1. The additive may be referred to herein as “the additive of the present invention”, “the ester compound” or as “the ester additive”. The skilled person would understand that the reaction of component a) and component b) may form a mixture of ester compounds and therefore that the ester additive may be a mixture of ester compounds. For example, the one or more ester compounds formed by the reaction of component a) and component b) may be monoester compounds, diester compounds or mixtures of monoester compounds and diester compounds. The mixture of ester compounds obtained may depend on the type and relative amounts of components a) and b) used, as further discussed below. Component a) an optionally substituted polycarboxylic acid or an anhydride thereof The additive of the present invention is prepared from component a) an optionally substituted polycarboxylic acid or anhydride thereof. In some embodiments the polycarboxylic acid or anhydride is unsubstituted. Suitably the polycarboxylic acid or anhydride is substituted, suitably with a hydrocarbyl group. In preferred embodiments the additive is prepared from a hydrocarbyl substituted polycarboxylic acid or an anhydride thereof. 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 (including 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 embodiments, the polycarboxylic acid or anhydride thereof is substituted with a Ci to C500 alkyl or alkenyl group, preferably a C2 to C100 alkyl or alkenyl group, preferably a Ce to C50 alkyl or alkenyl group, preferably a Cs to C40 alkyl or alkenyl group, more preferably a C10 to C38 alkyl or alkenyl group, preferably a C14 to C36 alkyl or alkenyl group, suitably a C14 to C32 alkyl or alkenyl group, for example a C14 to C28 alkyl or alkenyl group, preferably said alkenyl groups. In such embodiments, the substituted polycarboxylic acid or anhydride thereof may be formed by reacting the polycarboxylic acid or anhydride thereof with a suitable alkene to provide the alkenyl group as defined above. In some preferred embodiments the alkene is an internal olefin. The term internal olefin is used to refer to any alkene compound in which the alkene group is not terminal. 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 151810 available from Shell and Alphaplus C20-24. 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 one embodiment an internal olefin may be a p-olefin. Internal olefins may be prepared by isomerisation of an a-olefin. In some embodiments the polycarboxylic acid or anhydride thereof is substituted with a hydrocarbyl group derived from an a-olefin having 6 to 36 carbon atoms; preferably from 10 to 30 carbon atoms, suitably from 12 to 24 carbon atoms, for example from 14 to 18 carbon atoms. In some embodiments the polycarboxylic acid or anhydride thereof is substituted with a hydrocarbyl group derived from an internal olefin having 6 to 36 carbon atoms preferably from 10 to 32 carbon atoms, suitably from 12 to 24 carbon atoms, for example 14 to 18 carbon atoms. The optionally substituted polycarboxylic acid or anhydride thereof of component a) may be selected from pyromellitic acid, maleic acid, fumaric acid, oxalic acid, malonic acid, pimelic acid, suberic acid, glutaric acid, adipic acid, phthalic acid, succinic acid, citric acid, azelaic acid, sebacic acid and dimerised fatty acids, or anhydrides thereof. In one embodiment, component a) is an optionally substituted polycarboxylic acid or anhydride thereof selected from pyromellitic acid, malonic acid, sebacic acid and succinic acid. Suitably the optionally substituted polycarboxylic acid or anhydride thereof is an optionally substituted succinic acid or an anhydride thereof. Suitably component a) is a succinic acid or an anhydride thereof substituted with a hydrocarbyl group. In preferred embodiments, component a) is a polycarboxylic acid or anhydride thereof selected from pyromellitic acid, malonic acid, sebacic acid and succinic acid, substituted with a Ci to C500 alkyl or alkenyl group, preferably a C2 to C100 alkyl or alkenyl group, preferably a Ce to C50 alkyl or alkenyl group, preferably a Cs to C40 alkyl or alkenyl group, more preferably a C10 to C38 alkyl or alkenyl group, preferably a C14 to C36 alkyl or alkenyl group, suitably a C14 to C32 alkyl or alkenyl group, for example a C14 to C28 alkyl or alkenyl group. Suitably component a) is an optionally substituted succinic acid or an anhydride thereof substituted with such an alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group. Preferred acids are dicarboxylic acids. Thus preferably the additive of the invention is the reaction product of a) a hydrocarbyl substituted dicarboxylic acid or hydrocarbyl substituted anhydride thereof and b) a polyhydric alcohol of formula H-(OR)n-OR1, as defined herein. Suitable dicarboxylic acids include maleic acid, glutaric acid, fumaric acid, oxalic acid, malonic acid, pimelic acid, suberic acid, adipic acid, phthalic acid, succinic acid, azelaic acid, sebacic acid and dimerised fatty acids. In some embodiments, component a) is a dimerised fatty acid. Such compounds are formed from the dimerization of unsaturated fatty acids, for example unsaturated fatty acids having 6 to 50, suitably 8 to 40, preferably 10 to 36, for example 10 to 20 carbon atoms, or 16 to 20 carbon atoms. Such dimerised fatty acids may have 12 to 100 carbon atoms, preferably 16 to 72 carbon atoms such as 20 to 40 carbon atoms for example 32 to 40 carbon atoms. These compounds are well known in the art, particularly for their use as corrosion inhibitors. Particularly preferred dimerised fatty acids are mixtures of C36 dimer acids such as those prepared by dimerising oleic acid, linoleic acid and mixtures comprising oleic and linoleic acid, for example, tall oil fatty acids. In some embodiments, component a) is phthalic acid or an anhydride thereof, having the formula (A1)or(A2): (A1) (A2) wherein each of Rw, Rx, Ry and Rz is independently hydrogen or an optionally substituted hydrocarbyl group. Preferably each is hydrogen or an optionally substituted alkyl or alkenyl group. Preferably three of Rw, Rx, Ry and Rz are hydrogen and the other is an optionally substituted Ci to C500 alkyl or alkenyl group, preferably a C2 to C100 alkyl or alkenyl group, preferably a Ce to C50 alkyl or alkenyl group, preferably a Cs to C40 alkyl or alkenyl group, more preferably a C10 to C36 alkyl or alkenyl group, preferably a C14 to C32 alkyl or alkenyl group, suitably a C14 to C28 alkyl or alkenyl group. The alkyl or alkenyl group may be straight chain or branched. Preferably Rw, Rx and Rz are hydrogen and Ry is an optionally substituted alkyl or alkenyl group. In some preferred embodiments, component a) is an optionally substituted succinic acid or anhydride thereof of formula (I): R2-(Suc)x wherein R2 is hydrogen or an optionally substituted hydrocarbyl group; Sue represents the succinic acid or anhydride thereof; and wherein 1 <x <2. Suitably R2 is an optionally substituted Ci to C500 alkyl or alkenyl group, preferably a C2 to C100 alkyl or alkenyl group, preferably a Ce to C50 alkyl or alkenyl group, preferably a Cs to C40 alkyl or alkenyl group, more preferably a C10 to C38 alkyl or alkenyl group, preferably a C14 to C36 alkyl or alkenyl group, suitably a C14 to C32 alkyl or alkenyl group, for example a C14 to C28 alkyl or alkenyl group. Suitably R2 is a C14 to C28 alkyl or alkenyl group. The compound of formula (I) is suitably prepared by reacting maleic anhydride with a suitable alkene to provide the R2 group as defined above. The product of this reaction also includes a double bond. The maleic anhydride is present in the resultant molecule as a succinic acid moiety. This initial product may be described as a monomaleated compound, which may be a compound according to formula (I) wherein x = 1. The monomaleated compound may comprise an optionally substituted succinic acid or anhydride thereof of formula (A3) or (A4): (A3) (A4) wherein R2 is hydrogen or an optionally substituted hydrocarbyl group. Preferably R2 is an optionally substituted alkyl or alkenyl group. The double bond in the monomaleated product, e.g. within the R2 group of the compounds (A3) or (A4), can react with a further molecule of maleic anhydride to form a bismaleated compound, which may be a compound according to formula (I) wherein x = 2. Such bismaleated compounds may be represented by the structures (A5) or (A6): (A5) (A6) The skilled person would understand that such bismaleated products may comprise compounds with different structures depending on the position of the double bond in the R2 group of the monomaleated compound and the position on the R2 group which the second succinic acid or anhydride moiety is bonded to. Structures (A7) and (A8) are examples of possible structures of such bismaleated compounds which may be formed by said reaction, wherein R2 is the remainder of the group R2 as defined above, considering the four carbon atoms of the R2 which are shown in the structures below linking the succinic acid moieties. The double bond remaining in the compound may have any one of the positions indicated by the dotted lines. Thus it is possible to provide a hydrocarbyl group which is substituted with more than one succinic acid moiety. Depending on the extent of the bismaleation reaction discussed above, component a) may be a mixture of monomaleated and bismaleated compounds and therefore may be represented by formula (I) wherein x is greater than 1 and less than 2. In embodiments wherein component a) contains only monomaleated compounds, x is 1. In embodiments wherein component a) contains only bismaleated compounds, x is 2. In some embodiments, component a) has the formula (I) wherein x is at least 1.2, suitably at least 1.5, at least 1.7 or at least 1.8. Suitably x is approximately 2. In such embodiments, component a) suitably has the formula (I): R2-(Suc)x wherein R2 is R2 is a C14 to C28 alkyl or alkenyl group; Sue represents the succinic acid or anhydride thereof; and wherein 1.2 <x <2. The skilled person will appreciate that the ester compounds used in the present invention typically comprise mixtures of compounds and may be prepared from a mixture of monomaleated and bismaleated compounds. The ester compounds 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 compound has one succinic acid moiety per module. A bismaleated compound has two succinic acid moieties per molecule. A mixture comprising monomaleated compound and bismaleated compound in a 1:1 molar ratio would comprise an average of 1.5 succinic acid moieties per molecule of compound. Component a) comprises an optionally substituted polycarboxylic acid or an anhydride thereof. Preferably component a) comprises a hydrocarbyl substituted succinic acid or anhydride thereof. This may be referred to as a hydrocarbyl substituted succinic acid derived acylating agent. The average number of succinic acid moieties per molecule of acylating agent is sometimes referred to in the art as “P value”, which can be calculated by known methods. Suitably the ester compounds is prepared from a hydrocarbyl substituted succinic acid derived acylating agent comprising on average from 1 to 2 succinic acid moieties per molecule. In some preferred embodiments the present invention may involve the use of ester compounds derived from hydrocarbyl substituted acylating agents which include an average of at least 1.2 succinic acid moieties per molecule. As the skilled person will appreciate, a single molecule cannot have 1.2 succinic acid moieties. What is meant by at least 1.2 succinic acid moieties is the mean number of succinic acid moieties per molecule of acylating agent as the sum of all the succinic acid moieties present in a sample divided by the total number of molecules of acylating agent having one or more succinic acid moieties present in the sample. Preferably the hydrocarbyl substituted succinic acid derived acylating agent comprises on average at least 1.3 succinic acid moieties per molecule, more preferably at least 1.4 succinic acid moieties per molecule or at least 1.5. Suitably the hydrocarbyl substituted succinic acid derived acylating agent comprises on average up to 2 succinic acid moieties per molecule, suitably up to 1.9, suitably up to 1.8 or up to 1.75 succinic acid moieties per molecule. Suitably the hydrocarbyl substituted succinic acid derived acylating agent comprises on average from 1.2 to 2 succinic acid moieties per molecule, suitably from 1.3 to 1.8 or from 1.5 to 1.75 succinic acid moieties per molecule. In some embodiments, the hydrocarbyl substituted succinic acid derived acylating agent comprises predominantly bismaleated compound. By succinic acid moiety we mean to include residues of succinic acid present in diacid or anhydride form. In some embodiments R2 is an optionally substituted Ci to C500 alkyl or alkenyl group, preferably a C2 to C100 alkyl or alkenyl group, preferably a Ce to C50 alkyl or alkenyl group, preferably a Cs to C40 alkyl or alkenyl group, more preferably a C10 to C38 alkyl or alkenyl group, preferably a C14 to C36 alkyl or alkenyl group, suitably a C14 to C32 alkyl or alkenyl group, for example a C14 to C28 alkyl or alkenyl group. In the embodiments discussed above, R2 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 R2 is an unsubstituted alkyl or alkenyl group as defined above. The substituted succinic acid or anhydrides may suitably be prepared by reacting maleic anhydride with an alkene. In some preferred embodiments the alkene is an internal olefin. In one embodiment the internal olefin may be a p-olefin. Internal olefins may be prepared by isomerisation of an a-olefin. In some embodiments, the R2 group has a molecular weight of from 100 to 5000, preferably from 300 to 4000, suitably from 450 to 2500, for example from 500 to 2000 or from 600 to 1500. In some embodiments, the substituted succinic acid or anhydride thereof may comprise a mixture of compounds including groups R2 of different lengths. In such embodiments any reference to the molecular weight of the group R2 relates to the number average molecular weight for the mixture. In some embodiments, R2is a polyisobutenyl group, preferably having a number average molecular weight of from 100 to 5000, preferably from 200 to 2400, suitably from 220 to 1400. In some embodiments, R2 is a polyisobutenyl group having a number average molecular weight of from 400 to 700. In some embodiments, R2 is a polyisobutenyl group having a number average molecular weight of from 180 to 400. In some embodiments, R2is a polyisobutenyl group having a number average molecular weight of from 800 to 1200. In some preferred embodiments, R2 is an alkyl or alkenyl group having 6 to 40 carbon atoms, preferably 10 to 38 carbon atoms, more preferably 14 to 36 carbon atoms, suitably 18 to 26 carbon atoms, for example 20 to 24 carbon atoms. In some preferred embodiments, R2 is an alkyl or alkenyl group having 6 to 40 carbon atoms, preferably 10 to 38 carbon atoms, more preferably 14 to 36 carbon atoms, suitably 14 to 20 carbon atoms, for example 16 to 18 carbon atoms. In some preferred embodiments, R2 is an alkyl or alkenyl group having from 14 to 18 carbon atoms. Suitably R2 is the residue of an olefin. In such embodiments, component a) (of formula (A3) or (A4) is suitably obtained by the reaction of maleic acid with an olefin having 6 to 40 carbon atoms, suitably 10 to 38 carbon atoms, suitably 14 to 36 carbon atoms, preferably 14 to 28. In some embodiments, R2 is the residue of an a-olefin having 6 to 36 carbon atoms; preferably from 10 to 30 carbon atoms, suitably from 12 to 24 carbon atoms, for example from 14 to 18 carbon atoms. In some embodiments, R2 is the residue of an internal olefin having 6 to 36 carbon atoms preferably from 10 to 32 carbon atoms, suitably from 12 to 24 carbon atoms, for example 14 to 18 carbon atoms. Commercial sources of substituted succinic acids and anhydrides may also contain mixtures of compounds, for example including different compounds with substituents having 20 to 24 carbon atoms. In preferred embodiments, component (a) is not a dicarboxylic acid compound of formula (I): or an anhydride thereof. Suitably component (a) is not a dicarboxylic acid selected from itaconic acid, itaconic anhydride, 2-methylene glutaric acid, 2-methylene glutaric anhydride, 2-methylene adipic acid, 2-methylene adipic anhydride and isomers and / or mixtures thereof. Suitably component (a) is not itaconic acid. In some preferred embodiments, the additive of the present invention is the reaction product of: a) a succinic acid or anhydride having a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; and b) an alcohol of formula H-(OR)n-OR1. In some preferred embodiments, the additive of the present invention is the reaction product of: a) a succinic acid or anhydride having polyisobutenyl substituent having a number average molecular weight of from 700 to 1300; and b) an alcohol of formula H-(OR)n-OR1. In some preferred embodiments, the additive of the present invention is the reaction product of: a) a succinic acid or anhydride of formula (I): R2-(Suc)x wherein R2 is a C14 to C28 alkyl or alkenyl group; Sue represents the succinic acid or anhydride thereof; and wherein 1.2 <x <2; and b) an alcohol of formula H-(OR)n-OR1. In some preferred embodiments, the additive of the present invention is the reaction product of: a) a succinic acid or anhydride of formula (I): R2-(Suc)x wherein R2 is polyisobutenyl substituent having a number average molecular weight of from 700 to 1300; Sue represents the succinic acid or anhydride thereof; and wherein 1.2 <x <2; and b) an alcohol of formula H-(OR)n-OR1. Component b) an alcohol of formula H-(OR)n-OR1 The alcohol of formula H(OR)nOR1 may take a number of forms. In some embodiments, R is an unsubstituted alkylene group. In such embodiments, R is suitably 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. In such embodiments, R may be an ethylene, propylene, butylene, pentylene, or hexylene group. When R has more than 2 carbon atoms any isomer may be present. Preferably R is an ethylene or a propylene group, most preferably a propylene group. In some embodiments, R is suitably a group of formula (CH2)x wherein x is from 2 to 12, preferably from 2 to 6. In such embodiments, R is suitably an optionally substituted alkylene group. R is suitably 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. In some preferred embodiments, R is preferably CRaRbCRcRd and the alcohol has the formula H-(OCRaRbCRcRd)nOH wherein each of Ra, Rb, Rc and Rd is independently hydrogen or an optionally substituted alkyl group. Preferably each Ra, Rb, Rcand Rdis independently selected from hydrogen or an optionally substituted alkyl group having 1 to 20, preferably 1 to 12, more preferably 1 to 4, for example 1 to 2 carbon atoms. Preferably each of Ra, Rb, Rcand Rdis independently selected from hydrogen and an unsubstituted alkyl group, preferably having 1 to 20 carbon atoms, suitably 1 to 12 carbon atoms, preferably 1 to 4 atoms, for example 1 or 2 carbon atoms. Preferably at least two of Ra, Rb, Rc and Rd are hydrogen, more preferably at least three of Ra, Rb, Rc and Rd are hydrogen. In some embodiments, Ra, Rb, Rc and Rd are all hydrogen and R is an ethylene group CH2CH2. In some embodiments, three of Ra, Rb, Rc, and Rd are hydrogen and the other is an unsubstituted alkyl group having 1 to 12, preferably 1 to 4, suitably 1 to 2 carbon atoms and most preferably 1 carbon atom. In some embodiments, polyhydric alcohols used to prepare the additive of the present invention are prepared from epoxides, preferably terminal epoxides. R may comprise a mixture of isomers. For example when R is propylene, the polyhydric alcohol may include moieties -CH2CH(CH3)- and -CH(CH3)CH2- in any order within the chain. R may comprise a mixture of different groups for example ethylene, propylene or butylene units. Block copolymer units are preferred in such embodiments. R is preferably an ethylene, propylene or butylene group. R may be an n-propylene or n-butylene group or an isopropylene or isobutylene group. For example R may be -CH2CH2-, -CH2CH(CH3)-, -CH2C(CH3)2, -CH(CH3)CH(CH3)- or -CH2CH(CH2CH3)-. Preferably R is ethylene or propylene. More preferably R is -CH2CH2- or -CH(CH3)CH2-. In some embodiments, n is at least 1. Preferably n is from 1 to 100, 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, preferably from 1 to 14. In some embodiments, n is from 4 to 10, for example from 6 to 8. In some embodiments, n is from 1 to 6, suitably from 2 to 5, for example 3 or 4. In some embodiments, n is from 8 to 16, for example from 11 to 14. In some embodiments, n is from 18 to 26, suitably from 20 to 24, for example 22 to 23. In embodiments in which n is more than 1 component b) may be a polyhydric alcohol or an ether thereof. In some embodiments, n is 1 and R1 is hydrogen. In such embodiments, the alcohol of component b) may be a diol. As R may be substituted, the alcohol may be a polyol. Suitably R is a straight chain or branched alkylene group, suitably having from 2 to 10 carbon atoms, suitably from 2 to 6 carbon atoms. In such embodiments wherein n is 1 and R1 is hydrogen, R may be a straight chain or branched alkylene group, suitably having from 2 to 10 carbon atoms, suitably from 2 to 6 carbon atoms. Therefore the alcohol of component b) may be selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol and neopentyl glycol. In such embodiments wherein n is 1 and R1 is hydrogen, the alcohol of formula H(OR)nOR1 may be an alkylene glycol, suitably wherein R is a straight chain or branched alkylene group, suitably having from 2 to 10 carbon atoms, suitably from 2 to 6 carbon atoms. Suitably the alcohol of component b) is ethylene glycol or propylene glycol, preferably ethylene glycol. In some embodiments of the alcohol of formula H-(OR)n-OR1, the group R is a hydroxyl substituted alkylene group and therefore the alcohol has more than 2 hydroxy groups. Such a hydroxyl substituted alkylene group may have 1,2 or more hydroxyl groups. For example in some embodiments in which n is 1, the alcohol H-(OR)n-OR1 may be glycerol, pentaerythritol or trimethylolpropane. In some preferred embodiments, in the alcohol of formula H-(OR)n-OR1, n is 1 or more and R1 is an optionally substituted hydrocarbyl group. In such embodiments, component b) is a glycol ether or a polyglycol ether. In such embodiments, component b) may be provided by an ether of a polyhydric alcohol. For example component b) may be an ether of: a polyethylene glycol, a polypropylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol or tripropylene glycol. In such embodiments wherein R1 is an optionally substituted hydrocarbyl group and n is a positive integer, R1 is suitably an optionally substituted alkyl, alkenyl or aryl group, suitably an optionally substituted alkyl or alkenyl group. Preferably R1 has from 4 to 50 carbon atoms, preferably 4 to 40 carbon atoms, more preferably from 10 to 30 carbon atoms. R1 may be straight chain or branched. Preferably R1 is straight chain. In such embodiments, R1 is suitably a substituted alkyl or alkenyl group, suitably a substituted alkyl group. Suitable substituents are hydroxy and ester groups. In some embodiments R1 is a 2-hydroxy alkyl, alkenyl or aryl group. Suitably R1 is an unsubstituted alkyl or alkenyl group. Preferably R1 is an alkyl group, preferably an unsubstituted alkyl group. In some embodiments, n is greater than 1 and R1 is hydrogen. In such embodiments, the alcohol of formula H(OR)nOR1 is suitably a polyalkylene glycol, with R as defined above. In such embodiments, n is suitably from 2 to 24, suitably from 2 to 10, from 2 to 6, preferably from 2 to 5, for example 3 or 4. In such embodiments, R is suitably a group of formula (CH2)x wherein x is from 2 to 12, preferably from 2 to 6, suitably ethylene or propylene. In such embodiments, component b) may be a polypropylene glycol having a number average molecular weight of 425. In some embodiments, component b) may be selected from triethylene glycol, tetraethyelene glycol, propylene glycol, dipropylene glycol and tripropylene glycol. In some embodiments, component b) is selected from ethylene glycol, propylene glycol and oligomers or polymers thereof. In some embodiments, component b) may be a polypropylene glycol having a number average molecular weight of 725. The skilled person will appreciate that commercial sources of such alcohols of formula H-(OR)n-OH will often contain mixtures of compounds, for example in which n may be between 6 and 10. In some preferred embodiments, R1 is hydrogen. In embodiments in which R1 is hydrogen, each alcohol of formula H(OR)nOR1 can react with one or two acid or anhydride molecules to form the additive. For example, in some embodiments component b) of formula H(OR)nOR1 is reacted with approximately 2 molar equivalents of polycarboxylic acid. Such additive products contain the residues of two acid moieties per molecule. The two acid moieties may be the same or different. In some embodiments both acid moieties are the same. In some embodiments the two acid moieties are different. Preferably, both acid moieties are the same. In some embodiments, the additive of the present invention may be prepared from the reaction of a polyhydric alcohol of formula H-(OR)n-OH with approximately one equivalent of a first polycarboxylic acid or anhydride thereof and approximately one equivalent of a second polycarboxylic acid or anhydride thereof. For example, an additive of the present invention may be prepared from the reaction of a polyhydric alcohol of formula H-(OR)n-OH with approximately one equivalent of a succinic acid or anhydride substituted with an alkyl or alkenyl group having 6 to 36 carbon atoms and one equivalent of a succinic acid or anhydride substituted with a polyisobutenyl group having a number average molecular weight of from 200 to 1300. In some embodiments n is 0 and therefore component b) may be provided by an alcohol of formula R1OH. In such embodiments, R1 is suitably an optionally substituted alkyl, alkenyl or aryl group, preferably having from 1 to 60, preferably from 10 to 40 carbon atoms. Preferably R1 is an optionally substituted alkyl group. In such embodiments, R1 is suitably an unsubstituted alkyl group. The alkyl group may be straight chained or branched. In some embodiments R1 is an optionally substituted alkyl group having 4 to 40, preferably 6 to 30, more preferably 10 to 20 carbon atoms. One suitable alcohol for component b) is tetradecanol. In such embodiments wherein component b) is provided by an alcohol of formula R1OH, R1 is suitably an alkyl or aryl group having 1 to 12, suitably 2 to 10 or 4 to 8 carbon atoms. Suitable such alcohols for use herein include benzyl alcohol, tetradecanol, butanol, 2-butanol, isobutanol, octanol, 2-ethylhexanol, hexanol, cyclohexanol, cyclooctanol, 2-propylheptanol, isopropanol, 2-ethyl-1-butanol and mixtures thereof. In one embodiment, the alcohol R1OH is selected from benzyl alcohol, tetradecanol, butanol, octanol, 2-ethylhexanol, hexanol, cyclohexanol, cyclooctanol, 2-propylheptanol, 2-ethyl-1-butanol and mixtures thereof. In such embodiments, R is preferably an unsubstituted alkyl or cycloalkyl group having 1 to 12, suitably 4 to 10 or 4 to 8 carbon atoms. R may be a straight chain, branched or cyclic alkyl group. In such embodiments, component b) is preferably selected from butanol, octanol, 2-ethylhexanol, hexanol, cyclohexanol, cyclooctanol, 2-ethy 1-1-butanol and mixtures thereof. One especially preferred alcohol for component b) is 2-ethylhexanol. Component b) may comprise a mixture of alcohols. Such a mixture may result from a source of alcohol comprising a mixture of isomers and / or a mixture of homologues. In some embodiments alcohols from different sources may be combined to provide a mixture. In one embodiment component b) comprises a mixture of 2-ethylhexanol and isopropanol. The skilled person will appreciate that commercial sources of alcohols of formula R1OH will often contain mixtures of compounds. In some preferred embodiments, the component b) is provided by an alcohol of formula H-(OR)n-OR1, wherein n is 0 or 1, R is a straight chain or branched alkylene group having from 2 to 10 carbon atoms and R1 is hydrogen or an alkyl or aryl group having 4 to 12 carbon atoms; wherein when n is 1, R1 is hydrogen. In some preferred embodiments, the alcohol component b) is provided by an alcohol of formula H-(OR)n-OR1, wherein either: n = 1, R is a straight chain or branched alkylene group, suitably having from 2 to 10 carbon atoms and R1 is hydrogen; or n = 0 and R1 is an alkyl or aryl group having 4 to 12 carbon atoms. In some preferred embodiments, the alcohol component b) is provided by an alcohol of formula H-(OR)n-OR1, wherein either: n = 1, R is a straight chain or branched alkylene group, suitably having from 2 to 6 carbon atoms and R1 is hydrogen; or n = 0 and R1 is a straight chain, branched or cyclic alkyl group having 4 to 12 carbon atoms; suitably wherein the R and / or R1 are unsubstituted. In some preferred embodiments, the alcohol component b) is provided by either: a diol selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol and mixtures thereof; or an alcohol selected from butanol, octanol, 2-ethylhexanol, hexanol, cyclohexanol, cyclooctanol, 2-ethyl-1-butanol and mixtures thereof. In some preferred embodiments, the alcohol component b) is provided by ethylene glycol or 2-ethylhexanol. In some preferred embodiments the additive of the present invention is the reaction product of: a) an optionally substituted polycarboxylic acid or anhydride thereof selected from pyromellitic acid, malonic acid, sebacic acid and succinic acid; and b) either: a diol selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol and mixtures thereof; or an alcohol selected from butanol, octanol, 2-ethylhexanol, hexanol, cyclohexanol, cyclooctanol, 2-ethyl-1-butanol and mixtures thereof. In some preferred embodiments the additive of the present invention is the reaction product of: a) a succinic acid or anhydride having a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; and b) an alcohol of formula H-(OR)n-OR1, wherein n is 0 or 1, R is a straight chain or branched alkylene group having from 2 to 10 carbon atoms and R1 is hydrogen or an alkyl or aryl group having 4 to 12 carbon atoms; wherein when n is 1, R1 is hydrogen. In some preferred embodiments the additive of the present invention is the reaction product of: a) a succinic acid or anhydride having a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; and b) an alcohol of formula H-(OR)n-OR1, wherein either: n = 1, R is a straight chain or branched alkylene group, suitably having from 2 to 10 carbon atoms and R1 is hydrogen; or n = 0 and R1 is an alkyl or aryl group having 4 to 12 carbon atoms; suitably wherein the R and / or R1 are unsubstituted. In some preferred embodiments the additive of the present invention is the reaction product of: a) a succinic acid or anhydride having a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; and b) an alcohol of formula H-(OR)n-OR1, wherein either: n = 1, R is a straight chain or branched alkylene group, suitably having from 2 to 6 carbon atoms and R1 is hydrogen; or n = 0 and R1 is a straight chain, branched or cyclic alkyl group having 4 to 12 carbon atoms; suitably wherein the R and / or R1 are unsubstituted. In some preferred embodiments the additive of the present invention is the reaction product of: a) a succinic acid or anhydride having a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; and b) either: a diol selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol and mixtures thereof; or an alcohol selected from butanol, octanol, 2-ethylhexanol, hexanol, cyclohexanol, cyclooctanol, 2-ethyl-1-butanol and mixtures thereof. In some preferred embodiments the additive of the present invention is the reaction product of: a) a succinic acid or anhydride having a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; and b) ethylene glycol or 2-ethylhexanol. In some preferred embodiments the additive of the present invention is the reaction product of: a) a succinic acid or anhydride having a C14 to Cis alkenyl group; and b) ethylene glycol; suitably wherein the Ci4to Cis alkenyl group is the residue of an internal olefin having from 14 to 18 carbon atoms. In such embodiments, component a) is suitably obtained by the reaction of maleic acid with an internal olefin having from 14 to 18 carbon atoms. In some preferred embodiments the additive of the present invention is the reaction product of: a) a succinic acid or anhydride of formula (I): R2-(Suc)x wherein R2 is a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; Sue represents the succinic acid or anhydride thereof; and wherein 1.2 <x <2; and b) an alcohol of formula H-(OR)n-OR1, wherein n is 0 or 1, R is a straight chain or branched alkylene group having from 2 to 10 carbon atoms and R1 is hydrogen or an alkyl or aryl group having 4 to 12 carbon atoms; wherein when n is 1, R1 is hydrogen. In some preferred embodiments the additive of the present invention is the reaction product of: a) a succinic acid or anhydride of formula (I): R2-(Suc)x wherein R2 is a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; Sue represents the succinic acid or anhydride thereof; and wherein 1.2 <x <2; and b) an alcohol of formula H-(OR)n-OR1, wherein either: n = 1, R is a straight chain or branched alkylene group, suitably having from 2 to 10 carbon atoms and R1 is hydrogen; or n = 0 and R1 is an alkyl or aryl group having 4 to 12 carbon atoms; suitably wherein the R and / or R1 are unsubstituted. In some preferred embodiments the additive of the present invention is the reaction product of: a) a succinic acid or anhydride of formula (I): R2-(Suc)x wherein R2 is a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; Sue represents the succinic acid or anhydride thereof; and wherein 1.2 <x <2; and b) an alcohol of formula H-(OR)n-OR1, wherein either: n = 1, R is a straight chain or branched alkylene group, suitably having from 2 to 6 carbon atoms and R1 is hydrogen; or n = 0 and R1 is a straight chain, branched or cyclic alkyl group having 4 to 12 carbon atoms; suitably wherein the R and / or R1 are unsubstituted. In some preferred embodiments the additive of the present invention is the reaction product of: a) a succinic acid or anhydride of formula (I): R2-(Suc)x wherein R2 is a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; Sue represents the succinic acid or anhydride thereof; and wherein 1.2 <x <2; and b) either: a diol selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol and mixtures thereof; or an alcohol selected from butanol, octanol, 2-ethylhexanol, hexanol, cyclohexanol, cyclooctanol, 2-ethyl-1-butanol and mixtures thereof. In some preferred embodiments the additive of the present invention is the reaction product of: a) a succinic acid or anhydride of formula (I): R2-(Suc)x wherein R2 is a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; Sue represents the succinic acid or anhydride thereof; and wherein 1.2 <x <2; and b) ethylene glycol or 2-ethylhexanol. The ester additive of the present invention is the reaction product of: a) an optionally substituted polycarboxylic acid or an anhydride thereof; and b) an alcohol of formula H-(OR)n-OR1, wherein R is an optionally substituted alkylene group, R1 is hydrogen or an optionally substituted hydrocarbyl group and n is 0 or a positive integer; provided that n is not 0 when R1 is hydrogen. In some embodiments component a) comprises a compound of formula (I) where x is 1 and component a) and component b) are reacted in a molar ratio of from 5:1 to 1:20, preferably from 2:1 to 1:10, more preferably from 1:1 to 1:4 for example from 1.5:1 to 1:2.5. In some embodiments component a) comprises a compound of formula (I) where x is 1 and component a) and component b) 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. Suitably component a) comprises a compound of formula (I) where x is 1 and component a) and component b) are reacted in an approximately 1:1 molar ratio, for example from 1.2:1 to 1:1.2. In such embodiments, component b) is suitably an alcohol selected from one or more of butanol, octanol, 2-ethylhexanol, hexanol, cyclohexanol, cyclooctanol and 2-ethyl-1-butanol, for example 2-ethylhexanol. In some embodiments, component a) is reacted with an excess of component b), suitably wherein component b) is a diol selected from one or more of ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol and neopentyl glycol, for example ethylene glycol. In some embodiments, component a) is reacted with from 0.5 to 1.5 molar equivalents of component b) per succinic acid or anhydride group present in component a). Suitably component a) is reacted with from 0.75 to 1.25 molar equivalents of component b) per succinic acid or anhydride group present in component a), suitably from 0.9 to 1.1 molar equivalents of component b), suitably approximately 1 molar equivalent of component b), per succinic acid or anhydride group present in component a). In some preferred embodiments, the ester additive is the reaction product of a substituted succinic acid or succinic anhydride and an alcohol of component b). In such embodiments, the additive preferably comprises compounds having the formula (C1) or (C2), suitably as major components: (C1) (C2) wherein R and R1 are as defined above in relation to component b) and R2 is as defined above in relation to compounds (A3) and (A4). Therefore R is an optionally substituted alkylene group, R1 is hydrogen or an optionally substituted hydrocarbyl group and n is 0 or a positive integer; provided that n is not 0 when R1 is hydrogen; and R2 is hydrogen or an optionally substituted hydrocarbyl group. Preferably R2 is an optionally substituted alkyl or alkenyl group. Suitably R2 is an optionally substituted Ci to C500 alkyl or alkenyl group, preferably a C2 to C100 alkyl or alkenyl group, preferably a Ce to C50 alkyl or alkenyl group, preferably a Cs to C40 alkyl or alkenyl group, more preferably a C10 to C38 alkyl or alkenyl group, preferably a C14 to C32 alkyl or alkenyl group, for example a C14 to C28 alkyl or alkenyl group. Preferably R2 is an unsubstituted C14 to C28 alkyl or alkenyl group, which may be branched or straight chain. Preferably, in the compounds of formula (C1) and (C2), either: n = 1, R is a straight chain or branched alkylene group, suitably having from 2 to 10 carbon atoms and R1 is hydrogen; or n = 0 and R1 is an alkyl or aryl group having 4 to 12 carbon atoms; suitably wherein the R and / or R1 are unsubstituted. Preferably either: n = 1, R is a straight chain or branched alkylene group, suitably having from 2 to 6 carbon atoms and R1 is hydrogen; or n = 0 and R1 is a straight chain, branched or cyclic alkyl group having 4 to 12 carbon atoms; suitably wherein the R and / or R1 are unsubstituted. In some embodiments, the ester additive comprises compounds having the formula (C3) or (C4), suitably as major components: wherein R2 is a C14 to C28 alkyl or alkenyl group, preferably an unsubstituted C14 to C28 alkyl or alkenyl group. In some embodiments, the ester additive comprises compounds having the formula (C5) or (C6): wherein R, R1 and R2are as defined above. In some embodiments, the ester additive comprises compounds having the formula (C8) and / or (C9) and / or regioisomers of said compounds and / or anhydride analogues of said compounds, or mixtures thereof: (C9) wherein R2 is a C14 to C28 alkyl or alkenyl group, preferably an unsubstituted C14 to C28 alkyl or alkenyl group. The additives may also include oligomers or polymers having a structure of formula (D): (D) wherein m is at least 1 and one of each X and Y is R2 and the other is hydrogen, and Z is hydrogen or COCHXCHYCOOH. In some embodiments in which R1 is hydrogen the acid / anhydride are reacted with the alcohol in an approximate 2:1 molar ratio and the additive includes compounds of formula (E1), (E2) or (E3): (E1) (E2) (E3) and mixtures and / or isomers thereof; wherein R and R2 are as defined above. In some embodiments each acid residue in formulae (E1), (E2) or (E3) is the same. In some embodiments the acid residues are different. In some embodiments, the diesel fuel composition comprises from 0.1 to 10000 ppm, preferably from 1 to 1000 ppm, preferably from 5 to 250 ppm, for example 5 to 150 ppm of the ester additive. In such embodiments, use of the first aspect and / or the method of the second aspect and / or the diesel fuel composition of the third aspect preferably involves reducing the impact of deposits in an exhaust gas recirculation system of a diesel engine when combusting said diesel fuel composition. Preferably the use and / or method reduces the formation of deposits in the EGR system. The ester additive may be added to diesel fuel at any convenient place in the supply chain. For example, the ester additive may be added to fuel at the refinery, at a distribution terminal or after the fuel has left the distribution terminal. If the ester 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 ester 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 ester additive in small bottles suitable for direct addition to fuel storage tanks or vehicle tanks. By diesel fuel we include any fuel suitable for use in a diesel engine either for road use or non-road 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 or a 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 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 cetane number of between 50 and 90, preferably between 55 and 90, more preferably between 60 and 85. Cetane number is suitably measured by the standard test method set out in IP 498. Preferably the 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 mm2s1, preferably from 2 to 15 mm2S'1, more preferably from 2 to 10 mm2s1, most preferably from 2 to 4.5 mm2s1. 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 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 of 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 10:99, 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 may comprise greater than 4 wt%, preferably greater than 5 wt%, of C14 to C16 n-alkanes. The renewable diesel may comprise less than 8 wt%, preferably less than 6 wt%, of C14 to C16 n-alkanes. The renewable diesel may comprise from 4 to 8 wt%, preferably from 5 to 6 wt%, of C14 to C16 n-alkanes. The renewable diesel may comprise greater than 5 wt%, preferably greater than 7 wt%, more preferably greater than 10 wt%, of C14 to C18 n-alkanes. The renewable diesel may comprise less than 20 wt%, preferably less than 18 wt%, more preferably less than 16 wt%, of C14 to C18 n-alkanes. The renewable diesel may comprise from 5 to 20 wt%, preferably from 7 to 18 wt%, more preferably from 10 to 16 wt%, of C14 to C18 n-alkanes. The renewable diesel may comprise from 3 to 30 wt% of C6 to C24 n-alkanes (i.e. n-paraffin). 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 5wt%, 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%. Preferably the renewable diesel is a hydrotreated triglyceride oil, suitably an HVO, having cetane number of between 50 and 90, preferably between 55 and 90, more preferably between 60 and 85 (according to IP 498); a cloud point of less than 10°C, preferably less than -5°C (according to IP 219); and a kinematic viscosity at 40°C of from 1 to 20 mm2s'1, preferably from 2 to 10 mm2s'1 (according to ASTM 445). Preferably the renewable diesel is a hydrotreated triglyceride oil, suitably an HVO, having an initial boiling point and a final boiling point within the range 265 to 380°C, preferably 290 to 375°C and a boiling range of less than 80°C, preferably 30 to 60°C. The diesel fuel composition used in the present invention suitably comprises 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 preferred 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 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. In addition to metal-containing contamination which may be present in diesel fuels there are circumstances where metal-containing species may deliberately be added to the fuel. For example, as is known in the art, metal-containing fuel-borne catalyst species may be added to aid with the regeneration of particulate traps. The presence of such catalysts may also give rise to injector deposits when the fuels are used in diesel engines having high pressure fuel systems. Metal-containing contamination, depending on its source, may be in the form of insoluble particulates or soluble compounds or complexes. Metal-containing fuel-borne catalysts are often soluble compounds or complexes or colloidal species. In some embodiments, the diesel fuel may comprise metal-containing species comprising a fuel-borne catalyst. Preferably, the fuel borne catalyst comprises one or more metals selected from iron, cerium, platinum, manganese, Group I and Group II metals e.g., calcium and strontium. Most preferably the fuel borne catalyst comprises a metal selected from iron and cerium. 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. The diesel fuel compositions used in 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. Suitable cetane number improvers may be selected from C2-24 alkyl nitrates and dialkyl peroxides, preferably decyl nitrate, 2-ethylhexyl nitrate and di-tert-butyl peroxides. Such cetane number improvers are suitably used at a concentration between 50 and 6,000 ppm, preferably between 50 and 750 ppm, based on the diesel fuel composition. In some embodiments of the method, use or diesel fuel composition of the present invention, the diesel fuel composition comprises one or more detergents, suitably nitrogen containing detergents. Suitable detergents may be selected from the following or mixtures thereof: (i) a quaternary ammonium salt additive; (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 carboxylic acid-derived acylating agent and hydrazine; (v) a salt formed by the reaction of a carboxylic acid with di-n-butylamine or tri-n-butylamine; (vi) 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 (vii) a substituted polyaromatic detergent additive. Preferably one or more further detergents are selected from one or more of: (i) a quaternary ammonium salt additive; (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. To form the quaternary ammonium salt additive (i) the nitrogen-containing species having a tertiary amine group is reacted with a quaternising agent. An especially preferred quaternary ammonium salt for use herein is formed by reacting methyl salicylate or dimethyl oxalate with the reaction product of a polyisobutylene-substituted succinic anhydride having a PIB number average molecular weight of 700 to 1300 and dimethylaminopropylamine. In some especially preferred embodiments the diesel fuel composition comprises a quaternary ammonium salt additive (i) 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 (i) 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 some embodiments the quaternary ammonium compounds are the quaternised reaction product of a fatty acid (for example oleic acid) and dimethylaminoproyl amine. 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 (C) or (D): R2 N--X--NHR4 R3 [O(CH2)m]nOH R3 (D) wherein R2 and R3 are the same or different alkyl, alkenyl, aryl, alkaryl or aralkyl groups having from 1 to 22 carbon atoms; X is a bond or an optionally substituted alkylene group having from 1 to 20 carbon atoms; n is from 0 to 20; m is from 1 to 5; and R4is hydrogen or a Ci to C22 alkyl group. In some preferred embodiments the compound of formula (C) is selected from 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. In some preferred embodiments the compound of formula (D) is selected from 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 (C) is N,N-dimethyl-1,3-diaminopropane (dimethylaminopropylamine) . To form the quaternary ammonium salt detergent (i) 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. 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. Preferred quaternising agents for use herein 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. An especially preferred quaternary ammonium salt (i) for use herein is formed by reacting methyl salicylate or dimethyl oxalate with the reaction product of a polyisobutylene-substituted 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. The amine used to prepare the Mannich detergent 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 detergent additive may be substituted with 0 to 4 groups on the aromatic ring (in addition to the phenol OH). For example it may be a hydrocarbyl-substituted cresol. Most preferably the phenol component is a monosubstituted phenol. Preferably it is 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. 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. The present invention reduces the impact of deposits in the EGR system and / or the post combustion system of a diesel engine. The diesel engine may be a direct injection diesel engine or an indirect injection diesel engine. In some embodiments the engine may be an off-road engine, for example a marine, rail or stationary engine. Stationary engines include engines for power generation and pumping. Most preferably the engine is a direct injection diesel engine. The ester additives used in the present invention have been found to be particularly effective in modern diesel engines having a high-pressure fuel system. Suitably the ester additives of present invention may be used to reduce the formation or deposits in the post combustion system of a diesel engine having a high-pressure fuel system. Suitably the diesel engine has a fuel pressure in excess of 1350 bar (1.35 x 108 Pa). It may have a pressure of up to 2000 bar (2 x 108 Pa) or more. 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 the 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 500 pm, preferably less than 200 pm, more preferably less than 150 pm, preferably less than 100 pm, most preferably less than 80 pm 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 of 250°C. Such modern diesel engines may be characterised by 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. Two non-limiting examples of such high-pressure fuel systems are: the common rail injection system, in which the fuel is compressed utilizing a high-pressure pump that supplies it to the fuel injection valves through a common rail; and the unit injection system which integrates the high-pressure pump and fuel injection valve in one assembly, achieving the highest possible injection pressures exceeding 2000 bar (2 x 108 Pa). In both systems, in pressurising the fuel, the fuel gets hot, often to temperatures around 100°C, or above. Preferably, the diesel engine has fuel injection system which comprises a common rail injection system. In common rail systems, the fuel is stored at high pressure in the central accumulator rail or separate accumulators prior to being delivered to the injectors. Often, some of the heated fuel is returned to the low pressure side of the fuel system or returned to the fuel tank. In unit injection systems the fuel is compressed within the injector in order to generate the high injection pressures. This in turn increases the temperature of the fuel. In both systems, fuel is present in the injector body prior to injection where it is heated further due to heat from the combustion chamber. The temperature of the fuel at the tip of the injector can be as high as 250 - 350 °C. Thus the fuel is stressed at pressures from 1350 bar (1.35 x 108 Pa) to over 2000 bar (2 x 108 Pa) and temperatures from around 100°C to 350°C prior to injection, sometimes being recirculated back within the fuel system thus increasing the time for which the fuel experiences these conditions. The EGR system includes a cooler. This component lowers the temperature of the recirculated exhaust gases. Exhaust gases enter the EGR system after they pass through or are generated within the combustion chamber. The exhaust gases may contain materials resulting from incomplete combustion. These materials may deposit within the EGR system. One component where deposit build up frequently occurs is in the cooler of the EGR system. Previously these deposits have not been studied in the same level of detail as other fuel system or combustion deposits. Indeed the finding of such deposits appears to be a relatively recent phenomenon. Particularly strong reviews of the work done to elucidate how they may form and why they are a problem can be found in Lance et al International Journal of Heat and Mass Transfer 126, (2018), 509-520 and SAE 2014-01-0629. The present inventors have studied the nature of the deposits found in the EGR system and in particular within the cooler. It has been surprisingly found that the formation of these deposits in particular can be reduced by the addition of one or more ester compounds as defined herein into the diesel fuel combusted in the engine. The use of the first aspect and / or the method of the second aspect and / or the diesel fuel composition of the third aspect may involve reducing the impact of deposits in the exhaust gas recirculation system of a diesel engine which is a high pressure, low pressure, hybrid or dedicated EGR system. Suitably the exhaust gas recirculation system is a high pressure, hybrid or dedicated EGR system. Preferably the exhaust gas recirculation system is a high pressure EGR system. The high pressure EGR may be either a stand-alone high pressure EGR system or part of a hybrid or a dedicated EGR system. Preferably the use, method or composition of the present invention reduces the formation of deposits in a high pressure EGR system of a diesel engine. By reducing the formation of deposits in an EGR system we mean that when a fuel comprising the EGR deposit reducing additive is combusted in an engine, a reduced level of deposits is obtained compared to when an otherwise identical fuel is combusted under identical conditions except for the inclusion of the ester additive. Suitably, a reduced level of deposits is a reduced mass of deposits in the EGR system or in a part of the EGR system. Therefore, reducing the formation of deposits in an EGR system may involve a reduction in the mass of the deposits formed when the diesel fuel is combusted in an engine compared to when an otherwise identical fuel is combusted under identical conditions except for the inclusion of the ester additive. The percentage reductions in the formation of deposits described herein may be percentage reductions in the mass of said deposits formed, which may be determined as described herein. The reference to identical conditions suitably includes the use of the same engine, the same fuel type and composition (apart from the additive defined herein) and the same operating conditions of the engine as discussed herein. Suitably addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine reduces the formation of deposits in an EGR system by at least 5%, preferably by at least 10%, for example at least 15% or at least 20%. In some embodiments the addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine may reduce the formation of deposits in an EGR system by at least 30%, for example at least 40% or at least 50%. Suitably addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine reduces the formation of deposits in the cooler an EGR system by at least 5%, preferably by at least 10%, for example at least 15% or at least 20%. In some embodiments the addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine may reduce the formation of deposits in the cooler of an EGR system by at least 30%, for example at least 40% or at least 50%. The reduction in deposits in an EGR system may be measured by any suitable means. One simple means by which the level of deposits in an EGR system may be determined is by weighing the system before and after use. One or more parts of the system may be weighed. Preferably the present invention reduces the total amount of deposits formed in an EGR system by at least 5%, preferably at least 10%, more preferably at least 15%, for example at least 20% or at least 30%. Suitably the present invention reduces the total amount of deposits formed within the cooler of an EGR system. Preferably the present invention reduces the total amount of deposits formed within the cooler of an EGR system by at least 5%, preferably at least 10%, more preferably at least 15%, for example at least 20% or at least 30%. The deposits that form in the EGR system may be analysed. This may be achieved, for example by extracting the deposits or a portion thereof into a solvent. The sample may be separated into soluble and non-soluble fractions; these may then be separately analysed by methods known to those skilled in the art, for example elemental analysis, thermogravimetric analysis and / or gas chromatography mass spectrometry. The deposits that form in the EGR system may be analysed for example by thermogravimetric analysis (TGA). A significant proportion of the deposits that occur on the cooler of an EGR system were found to be carbonaceous deposits that degrade at temperatures of between 400 to 540°C when subjected to thermogravimetric analysis (TGA). Thermogravimetric analysis (or TGA) involves measuring the mass of a sample over time as it is heated. This technique is well known to the person skilled in the art and the selection of an appropriate method and suitable equipment will be within the competence of one skilled in the art. The post combustion system of diesel engines is provided to reduce the emission of pollutants such as particulates and harmful gases into the environment. The formation of deposits on parts of the post combustion system can reduce the efficiency of the system and lead to an increase in the emission of particulate deposits and / or harmful gases. In some embodiments, the impact of the deposits may be reduced by a change in the nature of deposits. Preferably the present invention reduces the formation of deposits in the post combustion system. By reducing the formation of deposits in a post combustion system we mean that when a fuel comprising one or more ester compounds as defined herein is combusted in an engine, a reduced level of deposits is obtained compared to when an otherwise identical fuel is combusted under identical conditions except for the inclusion of the post combustion deposit reducing additive. Suitably, a reduced level of deposits is a reduced mass of deposits in the post combustion system or in a part of the post combustion system. Therefore, reducing the formation of deposits in an post combustion system may involve a reduction in the mass of the deposits formed when the diesel fuel is combusted in an engine compared to when an otherwise identical fuel is combusted under identical conditions except for the inclusion of the ester additive. Suitably addition one or more ester compounds as defined herein into the diesel fuel combusted in an engine reduces the formation of deposits in one or more components of the post combustion system by at least 0.01%, preferably by at least 0.1%, for example at least 1% or at least 2%. In some embodiments the addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine may reduce the formation of deposits in one or more components of the post combustion system by at least 3%, for example at least 4% or at least 5%. By the post combustion system of a diesel engine we mean to refer to any part of the engine through which exhaust gases pass after finally leaving the combustion system. The post combustion system may comprise one or more components selected from a turbocharger, a diesel oxidation catalyst, a diesel particulate filter, a selective catalytic reduction unit and an ammonia oxidation unit. The post combustion system may include these components in any order and this may order vary from vehicle to vehicle. The present invention may reduce the impact of deposits in or on one or more of these components. In some embodiments addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine reduces the formation of deposits in the turbocharger, suitably by at least 0.01%, preferably by at least 0.1%, for example at least 1% or at least 2%. The present invention may reduce deposits on a fixed geometry turbocharger or on a variable geometry turbocharger. Variable geometry turbochargers having moving parts which are controlled by the engine management system. This allows the aspect ratio of the turbocharger to be changed to optimise performance at different speeds. The formation of deposits can lead to parts sticking. As a result the turbocharger will not provide the correct level of boost and may ultimately fail. The reduction of deposits on the turbocharger is therefore highly beneficial. Preferably the present invention reduces deposits on the turbine wheel of the turbocharger. In some embodiments the addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine may reduce the formation of deposits in the turbocharger by at least 3%, for example at least 4% or at least 5%. In some embodiments addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine reduces the formation of deposits in the diesel oxidation catalyst, suitably by at least 0.01%, preferably by at least 0.1%, for example at least 1% or at least 2%. The diesel oxidation catalyst typically comprises a ceramic support structure coated with metals such as palladium, platinum and / or rhodium. The formation of deposits in the diesel oxidation catalyst can lead to a reduction in flow rate through the catalyst and / or poisoning of the catalyst. In some embodiments the addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine may reduce the formation of deposits in the diesel oxidation catalyst by at least 3%, for example at least 4% or at least 5%. Suitably addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine reduces the formation of deposits in the diesel particulate filter, suitably by at least 0.01%, preferably by at least 0.1%, for example at least 1% or at least 2%. The diesel particulate filter is designed to capture from the exhaust gases particulates such as soot which are formed in the combustion chamber. These particulates collect on the filter and are burnt off at intervals by the increasing temperature of the exhaust gases and the injection of additional fuel. This process is known as filter regeneration. The present invention may increase the interval between regenerations. This can improve the fuel economy of the engine and reduce emissions. By a reduction in regenerations we mean to include a reduction in active, passive or parked regenerations of the diesel particulate filter. For example there may be a reduction in the number of regeneration events per 1000 km. In some embodiments the addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine may reduce the formation of deposits in the diesel particulate filter by at least 3%, for example at least 4% or at least 5%. In some embodiments addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine reduces the formation of deposits in the selective catalytic reduction unit, suitably by at least 0.01%, preferably by at least 0.1%, for example at least 1% or at least 2%. Selective catalytic reduction is used to remove NOx and other harmful gases from the exhaust stream and involves the use of ammonia as a reductant in the presence of a catalyst. The selective catalytic reduction unit comprises a porous ceramic support and a catalyst, typically comprising a metal or a zeolite. The formation of deposits on the selective catalytic reduction unit can lead to a reduction in flow rate through the unit and / or poisoning of the catalyst. In some embodiments the addition of ester compounds as defined herein into the diesel fuel combusted in an engine may reduce the formation of deposits in the selective catalytic reduction unit by at least 3%, for example at least 4% or at least 5%. In some embodiments addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine reduces the formation of deposits in the ammonia oxidation catalyst, suitably by at least 0.01%, preferably by at least 0.1%, for example at least 1% or at least 2%. The ammonia oxidation catalyst is used to oxidise any ammonia present in the exhaust gases after passing through the selective catalytic reduction unit. The formation of deposits on the ammonia oxidation catalyst can lead to a reduction in flow rate through the catalyst and / or poisoning of the catalyst. In some embodiments the addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine may reduce the formation of deposits in the ammonia oxidation catalyst by at least 3%, for example at least 4% or at least 5%. In some embodiments addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine reduces the formation of deposits on sensors within the post combustion system, suitably by at least 0.01%, preferably by at least 0.1%, for example at least 1% or at least 2%. Sensors may be present in the post combustion to measure temperature, pressure and / or concentrations of gases such as NOx in the exhaust gases. If deposits are present on or around the sensors they may be unable to function correctly or inaccurate measurements may be taken leading to incorrect information being provided to the engine management system. This can lead to poor performance of the engine. In some embodiments the addition of one or more ester compounds as defined herein into the diesel fuel combusted in an engine may reduce the formation of deposits on sensors within the post combustion system by at least 3%, for example at least 4% or at least 5%. The reduction in deposits in post combustion system may be measured by any suitable means. One simple means by which the level of deposits in a part of a post combustion system may be determined is by weighing the part of the system before and after use. One or more parts of the system may be weighed. Other less direct methods may also be used. For example, an improvement in fuel economy may indicate longer regeneration intervals on a diesel particulate filter. Preferably the present invention provides an improvement in fuel economy of at least 0.1%, preferably at least 0.5%, suitably at least 1%, for example at least 2%. The engine management system of a vehicle may be interrogated to assess the performance of components such as the turbocharger, the diesel particulate filter, the diesel oxidation catalyst and the selective catalytic reduction unit. The invention may result in fewer error messages being provided by the engine management system to a driver. An increase in the necessary maintenance intervals for catalytic components may also indicate improved performance due to deposit reduction. The deposits that form in the post combustion system may be analysed. This may be achieved, for example by extracting the deposits or a portion thereof into a solvent. The sample may be separated into soluble and non-soluble fractions; these may then be separately analysed by methods known to those skilled in the art, for example elemental analysis, thermogravimetric analysis and / or gas chromatography mass spectrometry. Thermogravimetric analysis (or TGA) involves measuring the mass of a sample over time as it is heated. This technique is well known to the person skilled in the art and the selection of an appropriate method and suitable equipment will be within the competence of one skilled in the art. When post composition deposits contain soot, the soot density can be measured, for example using an AVL483 microsoot sensor. Particle size of the soot can be measured by techniques known to these skilled in the art. The reduction of deposits in the post combustion system of a diesel engine according to the present invention offers significant benefits. These include, but are not limited to: an increase in power generation; an increase in torque; an increase in fuel economy; a reduction in emissions; a reduction in combustion chamber deposits; an acceleration improvement; driveability improvements; a reduction in cold start issues; lower soot formation; mitigation of lubricant degradation and / or performance loss; a reduction in maintenance; a reduction in diesel exhaust fluid and consumption e.g. urea consumption; reduction in wear on all post combustion components (including but not limited to the turbo charger, oxidation catalyst, DPF, SCR CAT, sensors, and injectors within the post combustion system); increased longevity of exhaust components; an increase in the maintenance period for the engine and / or post combustion components; and the protection of intake components downstream of the EGR, for example swirl flaps, throttles and the intake manifold (due to a reduction in the likelihood of blocking etc.). In some embodiments of the present invention one or more ester compounds as defined herein reduces the formation of deposits in an EGR system, preferably by at least 5%, for example by at least 20% or at least 50%. In some embodiments of the present invention one or more ester compounds as defined herein reduces the impact of deposits on one or more of a turbocharger, a diesel oxidation catalyst, a diesel particulate filter, a selective catalytic reduction unit and an ammonia oxidation catalyst. In some embodiments of the present invention one or more ester compounds as defined herein prevents and / or removes deposits on sensors within the post combustion system, for example deposits on NOx sensors, temperature sensors and / or pressure sensors. Any feature of the invention may be combined with any other feature as appropriate. 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. Examples Preparation of ester additive A Additive A, which comprises one or more ester compounds of the present invention, is the reaction product of a C16 to C18 alkenyl substituted succinic acid with at least 2 molar equivalents of ethylene glycol, wherein the C16 to C18 alkenyl substituted succinic acid is formed from a C16 to C18 internal olefin. Additive A was combined with A150 solvent to provide additive composition A containing 75% w / w active material of additive A. Preparation of ester additive B Additive B, which comprises one or more ester compounds of the present invention, was prepared as follows. Alphaplus C20-24 olefin (642.1 g, 2.08mol) was heated to 185°C and maleic anhydride (407.9 g, 4.16 mol, 2 equivalents) was added over a period of 3 hours. The mixture was then reacted for a further 9 hours at 215°C, then distilled under vacuum to provide 953.5 g of a deep brown liquid. Acid value was measured at 6.88 mmolH+ / g, as determined by titration with lithium methoxide. This corresponds to approximately 67% bismaleation of the olefin. The product obtained by the above process (bismaleated C20-24 ASA) (473.5 g, AV 6.88 mmolH+ / g,) was heated to 80°C. 2-ethylhexanol (211.9 g, 1 equivalent based on anhydride groups present) was added and stirred at 80°C for 15 mins. The temperature was increased to 165°C and mixture stirred at this temperature for approximately 4 hrs. The infra-red spectra was monitored until the anhydride absorption band was absent and then the temperature of the mixture was reduced to 80°C. Aromatic 150 solvent (289.7g) was added to provide the additive composition B (953.4 g) comprising 70% w / w active material of additive B. Engine Testing Engine testing was carried out as described below to assess the performance of the ester additives of the present invention in the reduction of deposits in the exhaust gas recirculation system and / or the post combustion system of a diesel engine when combusting a diesel fuel composition comprising the ester additives. Engine Details A Euro 6 compliant 2.0 litre, HSDI engine was connected to a test automation system and test bed 5 fitted with an engine dynamometer. The engine was controlled by an ECU supplied by the engine manufacturer. The engine had had over 1100h of use prior to the first test. The engine oil was changed prior to performing the first test. Modifications / Test Setup 10 1. No SCR Catalyst or associated components were present in the exhaust system. 2. High pressure EGR cooler is artificially controlled to 40°C for the duration of the test. The base fuel was an RF-06-03 diesel fuel (Haltermann Carless, UK) having the following 15 specification in Table 1. Table 1 Watere Units Results I Maximum iMtethna i Density 15X i®? :030.0 : 333,0 :837.0 SmW4O52 'WW (Rett)....................................................:........ ■.........i'Pass..........i-............................i-...................VISUAL................. CiUw'lfemter........................................................... 53.8....... 52 0................. :54.6....... iASTWbS13........... 8" i’C ’ ' :214.3 ‘ 1 ASTMD88..... 10» wRecoveredat....................................................:232.0.....................................|.....................ASTMOSS 50¾ wRecm^fudat X 275.5 '245.0 :- 1ASTMD88 »0\wRecovered at..... "W................:330.2'......... j ..... 95 - ^Recovered at......................................ix................:348.0.........:345.6......................:350 0........ ASIM D86 FBPt ..................................................... X............. 356.2...... -.......................... 376:6..... ASIM D88............. Aromatics by RA ........... .......... ......:%{W) 6IS’SXone^^ ......i' ASIM 01319........ OtefmsbyFIA :%tv?v) 5.5: : ■ Flash Pmn! Pensky Closed........................ "C............. 32 0............55.0.....................-...................iASWDOS........... SulphurComenf.............. .............. ..........ImgAg.........:« 36............1- ........HQ(G .......... '^1^05453'......... ViwcsrtyatWc............................................:mm2fe '":3X®2........Js^OG........................:6:300...............:AS™b445 .......... Xloua Point..................................................:X...............--IS.....................................................................;ASTMb25bO.......... XFPP .....:X .......;-20 .........:- .......Pis ........... WiTs :Luj)ric8y(WSD1,4)at6b‘C ...............W ......USQ .......:- ...................:400 ............ :180 12156-1 ......... iCardon Residue^ ......... Aifmfm) xb.lO -................... iOXO.......1ASTMD453G Wb................. ........... ................................p'Ooi.......6..... ......... .......:0.010 ......... .......... FAME Content: None Detected..........................;.........-...........:Pass...........:■................................i-................ EN 1407S............... Polytypic iWrisSt: Hydrocarbons ;ft(wnQ :5,8 3.6 5.6 :EN i29ie Total tomatic^ ........... .....:%(nvmj......22 2........: ........6 ........ EN12616 ■ Water Content : mgAg ■ 50 ’. : 20b , 1P 438 Water &Sediment........................ I-WOIO : ............;ASWt D27O8 Strong Add Number ;mj iOKOH / g :- 10.02 ASlVuC'4 Oxidation Stability ung «0.1 per ;2.5 ;ASW l>22~4 :100m! : : Copper Corrosion, 3htW 10QX...................:...... -...........i IB............................................:-................ ASiWiSO Oxygen Content:¾^ :^ 604 iELEN^FAL : Wrrpntiit Analysis iCarbbn Content........................ :86.89 ;ASTM:D5291 ; .............ASIM 05291 fH^ragen Content .........................113.11 ........iASTMD5291 : ASTMD5291 : Carders Wight Frsdion ........................... ‘ ............" ’ X......... Xateufatten"" i C / H Mass Ralio..................'....................'...........: '............lei?.............sCALCULAtiON........ ......................Calculation'' ^Atomic WC Raho.......................................................................iiWrb.......XALCULAWN.........i..........................Calculation AtoWRatio....... « 6:6663 XALCULAriON ..................Xaicufation :Gross Heat of Combustion"............... ..........:«JAg...........:451.72.........:IP 12......................... .....................iiP 12 : Net Heat of Combustion........... iMJAg :42.84 ' ilP 12...............i IP 12 Net Heat of Combustion btu / lfe 18460 'CALCULATION iCalcutalicii 5 Test Additives and Treat Rate The example ester additive compositions were dosed at 100mg / kg into the base diesel fuel described above, to provide the treat rates of the active material (ester additives A and B) noted in Table 2 and Table 3 below. The ester additive composition Awas dosed into the base fuel to provide 10 test fuel 1 and the ester additive composition B was dosed into the base fuel to provide test fuel 2. Method of Soot Deposition Measurement (DPF &EGR Soot Weight) The quantity of soot deposited in the diesel particulate filter (DPF) of the post combustion system of the engine or the quantity of soot deposited in the exhaust gas recirculation system (EGR) of the engine was established by weighing the relevant components before and after each test. Fuel 1 was tested for the effect on the quantity of soot deposited in the EGR compared to the base fuel. Fuel 2 was tested for the effect on the quantity of soot deposited in the DPF compared to the base fuel. The procedures for each test are similar and the combined details are provided below. Prior to the initial weighing, the DPF is passively regenerated on the test bed to remove any residual soot. Once the regeneration is complete, the DPF is placed into an oven, pre-heated to 185°C, affixed to a set of scales. The weight measurement was taken as an average over 15 minutes, once the scales had stabilised. This weighing process is repeated at the end of the test. The variance between the weight measured before and after the test represents the change in mass due to soot deposition. Test Procedure [D] EGR Cleaned and weighed [D] DPF + S / ave EGR Installation Engine Start + Warm-Up Passive DPF Regeneration by varying the engine speed and load until the regeneration is complete. The differential pressure across the is used to monitor the regeneration progress. Engine Stop Change to test fuel [C] DPF + Slave EGR Removal [C] DPF Start-of-Test (SOT) Weighing - [C] DPF + [C] EGR Installation Engine Start + Warm-Up 8-Hour Steady-State Test Cycle - 1200RPM - 60Nm Engine Stop [D] DPF Removal and End-of-Test (EOT) Weighing [D] EGR Removal and EGR End of Test Weighing [C] indicates a clean component [D] indicates a fouled component Results The results of the tests described above with Fuel 1 and Fuel 2 are shown in Tables 2 and 3, respectively. 5 Table 2 Test fuel Additive Treat rate of additive composition (mg / kg) Treat rate of additive (mg / kg) EGR Soot Weight [g] Base fuel - - 1.59 1 A 100 75 0.53 10 Table 3 Test fuel Additive Treat rate of additive composition (mg / kg) Treat rate of additive (mg / kg) DPF Soot Weight [g] Base fuel - - 36.35 2 B 100 70 34.02 These results demonstrate that the use of the ester additives described herein in a diesel fuel composition may provide a significant reduction in deposits in the post combustion system of a diesel 15 engine combusting said fuel, specifically a reduction in the soot deposited on the diesel particulate filter of the post combustion system, and / or a reduction in deposits in the exhaust gas recirculation system of a diesel engine combusting said fuel.

Claims

1. Use of one or more ester compounds as an additive in a diesel fuel composition to reduce the impact of deposits in the exhaust gas recirculation system and / or the post combustion system of a diesel engine when combusting said diesel fuel composition; wherein the one or more ester compounds are the reaction product of:a) an optionally substituted polycarboxylic acid or an anhydride thereof; andb) an alcohol of formula H-(OR)n-OR1, wherein R is an optionally substituted alkylene group, R1 is hydrogen or an optionally substituted hydrocarbyl group and n is 0 or a positive integer; provided that n is not 0 when R1 is hydrogen.

2. A method of reducing the impact of deposits in the exhaust gas recirculation system and / or the post combustion system of a diesel engine, the method comprising combusting in the engine a diesel fuel composition comprising as an additive one or more ester compounds; wherein the one or more ester compounds are the reaction product of:a) an optionally substituted polycarboxylic acid or an anhydride thereof; andb) an alcohol of formula H-(OR)n-OR1, wherein R is an optionally substituted alkylene group, R1 is hydrogen or an optionally substituted hydrocarbyl group and n is 0 or a positive integer; provided that n is not 0 when R1 is hydrogen.

3. A diesel fuel composition comprising as an additive one or more ester compounds; wherein the one or more ester compounds are the reaction product of:a) an optionally substituted polycarboxylic acid or an anhydride thereof; andb) an alcohol of formula H-(OR)n-OR1, wherein R is an optionally substituted alkylene group, R1 is hydrogen or an optionally substituted hydrocarbyl group and n is 0 or a positive integer; provided that n is not 0 when R1 is hydrogen;wherein combustion of the diesel fuel composition in a diesel engine reduces the impact of deposits in the exhaust gas recirculation system and / or the post combustion system of a diesel engine compared to when an otherwise identical diesel fuel without the additive is combusted under identical conditions.

4. The use, method or composition according to any preceding claim, wherein component a) is a hydrocarbyl substituted succinic acid or a hydrocarbyl substituted succinic anhydride.

5. The use, method or composition according to any preceding claim, wherein component a) is an optionally substituted succinic acid or anhydride thereof of formula (I):R2-(Suc)xwherein R2 is hydrogen or an optionally substituted hydrocarbyl group;Sue represents the succinic acid or anhydride thereof; and wherein 1 <x <2.

6. The use, method or composition according to claim 5, wherein R2 is a C14 to C28 alkyl or alkenyl group.

7. The use, method or composition according to claim 5 or claim 6, wherein x is at least 1.2.

8. The use, method or composition according to any one of claims 1 to 6, wherein component a)is an optionally substituted succinic acid or anhydride thereof of formula (A3) or (A4):(A3) (A4)wherein R2 is hydrogen or an optionally substituted hydrocarbyl group.

9. The use, method or composition according to any preceding claim, wherein in component b) R is ethylene or propylene, preferably -CH2CH2- or -CH(CH3)CH2-, more preferably -CH(CH3)CH2-; and n is from 1 to 30.

10. The use, method or composition according to any preceding claim, wherein in component b) n is 1, R1 is hydrogen and R is a straight chain or branched alkylene group, preferably wherein component b) is ethylene glycol or propylene glycol, preferably ethylene glycol.

11. The use, method or composition according to any one of claims 1 to 9, wherein in component b) n is 0 and R1 is an optionally substituted alkyl, alkenyl or aryl group having from 1 to 60 carbon atoms.

12. The use, method or composition according to claim 11, wherein R1 is an alkyl or aryl group having 1 to 12 carbon atoms, preferably selected from benzyl alcohol, tetradecanol, butanol, 2-butanol, isobutanol, octanol, 2-ethylhexanol, hexanol, cyclohexanol, cyclooctanol, 2-propylheptanol, isopropanol, 2-ethyl-1-butanol and mixtures thereof.

13. The use, method or composition according to any one of claims 1 to 9, wherein in component b) either:n = 1, R is a straight chain or branched alkylene group, preferably having from 2 to 10 carbon atoms and R1 is hydrogen; orn = 0 and R1 is an alkyl or aryl group having 4 to 12 carbon atoms.

14. The use, method or composition according to any one of claims 1 to 9, wherein component b) is either:a diol selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol and mixtures thereof; oran alcohol selected from butanol, octanol, 2-ethylhexanol, hexanol, cyclohexanol, cyclooctanol, 2-ethyl-1-butanol and mixtures thereof.

15. The use, method or composition according to any preceding claim, wherein component a) and component b) are reacted in a molar ratio of from 2:1 to 1:2, preferably wherein component b) is selected from one or more of benzyl alcohol, tetradecanol, butanol, 2-butanol, isobutanol, octanol, 2-ethylhexanol, hexanol, cyclohexanol, cyclooctanol, 2-propylheptanol, isopropanol, 2-ethyl-1-butanol and mixtures thereof.

16. The use, method or composition according to any one of claims 1 to 14, wherein component a) is reacted with an excess of component b), preferably wherein component b) is a diol selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol and mixtures thereof.

17. The use, method or composition according to any preceding claim, wherein the one or more ester compounds are the reaction product of:a) a succinic acid or anhydride having a C10 to C30 alkyl or alkenyl group, preferably a C14 to C28 alkyl or alkenyl group; andb) an alcohol of formula H-(OR)n-OR1, wherein either:n = 1, R is a straight chain or branched alkylene group, suitably having from 2 to 10 carbon atoms and R1 is hydrogen; orn = 0 and R1 is an alkyl or aryl group having 4 to 12 carbon atoms.

18. The use, method or composition according to any preceding claim, wherein the one or more ester compounds include compounds having the formula (C1) or (C2):(C1)(C2)wherein R is an optionally substituted alkylene group; R1 is hydrogen or an optionally substituted hydrocarbyl group; n is 0 or a positive integer; provided that n is not 0 when R1 is hydrogen; R2 is hydrogen or an optionally substituted hydrocarbyl group, preferably wherein R2 is a C14 to C28 alkyl or alkenyl group.

19. The use, method or composition according to any preceding claim wherein the one or more ester compounds are present in the fuel in an amount of from 10 to 200 ppm.

20. The use, method or composition according to any preceding claim which reduces the formation of deposits in the exhaust gas recirculation system of a diesel engine.

21. The use, method or composition according to any preceding claim which reduces the formation of deposits on a cooler of the exhaust gas recirculation system.

23. The use, method or composition according to any preceding claim, wherein the exhaust gas recirculation system is a high pressure exhaust gas recirculation system, wherein the high pressure EGR system is either a stand-alone high pressure EGR system or is part of a hybrid or a dedicated EGR system.

24. The use, method or composition according to any preceding claim, wherein the diesel fuel composition comprises one or more detergents.

25. The use, method or composition according to claim 23, wherein the one or more detergents are selected from the following or mixtures thereof:(i) a quaternary ammonium salt additive;(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 carboxylic acid-derived acylating agent and hydrazine;(v) a salt formed by the reaction of a carboxylic acid with di-n-butylamine ortri-n-butylamine;(vi) 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(vii) a substituted polyaromatic detergent additive.

25. The use, method or composition according to any preceding claim which reduces deposits in the post combustion system of a diesel engine having a pressure in excess of 1350 bar.

26. The use, method or composition according to any preceding claim which reduces the formation of deposits on the turbocharger of the post combustion system.

27. The use, method or composition according to any preceding claim which reduces the formation of deposits on the diesel oxidation catalyst of the post combustion system.

28. The use, method or composition according to any preceding claim which reduces the formation of deposits on the diesel particulate filter of the post combustion system.

29. The use, method or composition according to any preceding claim which reduces the formation of deposits on the selective catalytic reduction unit of the post combustion system.

30. The use, method or composition according to any preceding claim which reduces the formation of deposits on the ammonia oxidation catalyst of the post combustion system.

31. The use, method or composition according to any preceding claim which reduces the formation deposits on sensors within the post combustion system.

32. The use, method or composition according to any preceding claim which reduces the formation of deposits in one more components of the post combustion system by at least 5%.

33. The use, method or composition according to any preceding claim, wherein the diesel engine is an off-road engine, for example a marine, rail or stationary engine.

34. The use, method or composition according to any preceding claim which provides one or more benefits selected from: an increase in power generation; an increase in torque; an increase in fuel economy; a reduction in emissions; a reduction in combustion chamber deposits; an acceleration improvement; driveability improvements; a reduction in cold start issues; lower soot formation; mitigation of lubricant degradation and / or performance loss; a reduction in maintenance; a reduction in diesel exhaust fluid and consumption e.g. urea consumption; reduction in wear on all post combustion components (including but not limited to the turbo charger, oxidation catalyst, DPF, SCR CAT, sensors, and injectors within the post combustion system); increased longevity of exhaust components; and the protection of intake components downstream of the EGR, for example swirl flaps, throttles and the intake manifold (due to a reduction in the likelihood of blocking etc.).