This invention relates to fuel oil compositions, and to additives for use in such compositions. More especially it relates to diesel, heating, and jet fuel oil compositions, to improving cetane ratings and to reduction of particulate emissions on combustion.

Although modern internal combustion engines are highly efficient and give almost complete combustion of the hydrocarbon fuel used, the slight reduction from total efficiency leads to the formation of black smoke, a proportion of which is particulate carbon. Apart from the smoke's being unpleasant to breathe and unsightly, the carbon particles may have absorbed in them polynuclear hydrocarbons, which also result from incomplete combustion, some of which are known carcinogens.

It has been previously proposed to use certain additives to reduce smoke. These additives, which are based on metal salts, reduce smoke at the expense of increasing particulate emission, because the additive is emitted in the form of oxide or sulphate which contributes to the mass of particulates in the exhaust.

It has been observed that the use of a diesel fuel of low cetane number results in greater particulate emission compared with that from the same engine using a higher cetane number fuel, and numerous additives have

been proposed for increasing the cetane rating of diesel uels.

Commonly employed additives are nitrates, usually alkyl or cycloalkyl nitrates, isooctyl nitrate being typical of the additives frequently used. The molecular weight range of the most frequently used nitrates is such that they are volatile at room temperature. As they are used in relatively high concentrations in fuels, they tend to dominate additive blends, which also contain the corrosion inhibitors, antioxidants, dispersants, low temperature flow improvers, metal deactivators, antifoams and other additives commonly used to enhance the perform¬ ance or assist in storage stability of a typical diesel fuel. The volatility of the nitrates and their proper¬ ties are such that they may cause nausea, and very careful handling is required when they are being trans¬ ported, made up into additive blends, or added to diesel fuel.

It has previously been proposed in U.S. Patent No. 4 365 973 to improve the cold flow, cetane, and anti- icing properties of diesel fuel by an additive package in which the cetane improver component is an alkyl nitrate of from 4 to 16 carbon atoms, C ₅ to C^2 being preferred, but tridecyl, tetradecyl, pentadecyl, and hexadecyl nitrate are listed, though not employed in any example. In U.S. Patent No. 4 420 311 there is employed an optionally alkyl- or alkoxy-substituted cyclododecyl

nitrate to raise the cetane rating of a low cetane number diesel fuel. Cyclododecyl nitrate itself is found to be more effective on a mass basis than cyclohexyl nitrate.

In U.S. Patent No. 4 585 461 it is, however, stated that with an increase in molecular weight the nitrates become less effective for a given mass. Although in DE- OS-3233834 the nitrates of alcohols having up to 13 carbon atoms are disclosed for improving the cetane rating of diesel fuels obtained by hydrogenating coal, the examples employ only amyl, iso-propyl. cyclohexyl, and 2-ethylhexyl nitrates, confirming a preference for nitrates of lower molecular weight alcohols.

In EP-A-157 684, there is disclosed the use of a nitrate of the formula

R (OCH ₂ CHX) _n ON0 ₂ in which R is inter alia an alkyl radical with from 6 to 20 carbon atoms, X is a hydrogen atom or a methyl group, and n is from 1 to 20, preferably 2 to 10, as a diesel fuel cetane enhancer, the nitrate also being said to act as a detergent additive, and is shown to reduce the increase in particulate emissions from the beginning to the end of a 2500 km test run, although a slight increase in the initial particulate emissions, compared with the base fuel, is noted.

The present invention is based on the observation that certain alkyl nitrates give improvements in cetane

ratings in diesel fuels, coupled with a reduction in particulate emissions.

The present invention provides a fuel oil composi¬ tion comprising an alkyl nitrate, the alkyl group of which contains from 13 to 25 carbon atoms.

The invention also provides a fuel oil composition comprising an alkyl nitrate, the alkyl group of which contains from 13 to 25 carbon atoms, and an ashless dispersant, especially a macrocyclic polyamine dis¬ persant.

The invention further provides a fuel oil composi¬ tion comprising an alkyl nitrate, the alkyl group of which contains from 13 to 25 carbon atoms, an ashless dispersant, especially a macrocyclic polyamine disper¬ sant, and a foam inhibitor.

The invention, in a further aspect, provides an additive and additive concentrate containing the C^ ₃ to C ₂ 5 alkyl nitrate, and an ashless dispersant, especially a macrocyclic polyamine dispersant. Optionally, and preferably, the additive concentrate also contains a foam inhibitor.

The invention further provides a method of enhancing the cetane rating of a fuel oil, which comprises incor¬ porating in the fuel a C ₁₃ to C25 alkyl nitrate. Also provided is a method of reducing particulate emission from a diesel engine which comprises supplying to the engine a fuel oil comprising a C ₁₃ to C ₂ 5 alkyl nitrate in a proportion sufficient to reduce particulate emission

from the engine operated on such fuel. Further provided is the use of a C ₁₃ to C ₂ 5 alkyl nitrate as an additive in a fuel oil to reduce particulate emission. Still further provided are such methods and use wherein an ashless dispersant, especially a macrocyclic polyamine dispersant, is also employed. In such method and use providing reduced particulate emission, there may be two effects. One is that resulting from a reduction in injector nozzle fouling, such reduction including both the removal of existing deposits and the inhibition of deposit formation. The other is the direct effect of the combustion of a fuel containing the alkyl nitrate and, if present, the ashless dispersant rather than an otherwise identical fuel containing neither the alkyl nitrate nor the dispersant.

The invention finds valuable use in petroleum-based middle distillate fuel oils, more especially those having a cetane rating in excess of 45 in the absence of any cetane enhancer. It is also applicable to vegetable- based fuel oils, for example rape seed oil, used alone or in admixture with a petroleum distillate oil.

It is believed that alkyl nitrates act as cetane enhancers because on the decomposition of the nitrate group in the combustion chamber reactive hydroxyl radicals are formed by a radical reaction of nitrogen dioxide with a fuel hydrocarbon yielding nitrous acid, which in turn decomposes to nitrous oxide and the hydroxyl radical which facilitates the combustion

process. The hydroxyl radical is also postulated to oxidize any soot that may be formed during the later stages of combustion largely from the higher molecular weight less volatile fuel components that remain in fuel droplets after the more volatile components have evaporated. Based on the outline theory above, it would be expected that to provide a given number of hydroxyl radicals, and to achieve a given cetane rating improve¬ ment, a greater mass of a higher molecular weight alkyl nitrate would be needed than of the generally used octyl nitrate. Surprisingly, however, it has been found that the alkyl nitrates employed in the present invention are substantially as active as cetane enhancers on a mass basis as octyl nitrate, giving a significant advantage in that the lower proportion of nitrate present itself results in lower nitrogen oxide emissions.

A further advantage of the employment of the alkyl nitrates according to the invention is that their lower volatility makes their handling or the handling of additive concentrates containing them less difficult.

Although the applicants do not wish to be bound by any theory, it is believed that the enhanced molar activity as cetane enhancers of the alkyl nitrates of the present invention, and their activity in reducing particulate emissions, may result from their remaining in the fuel droplet, or staying associated with the less volatile components of the fuel, for longer in the

combustion process and accordingly still being present to yield hydroxyl radicals when the less volatile fuel components are burning.

Further, while the applicants again do not wish to be bound by any theory, it is believed that under given conditions (which include any deposits present in injectors or elsewhere upstream of the combustion chamber) the presence of the ashless dispersant in the fuel, or in the fuel/air mixture, in the combustion chamber results in an improvement in the quality of combustion, as measured by completeness of oxidation. This improvement may in turn be the result of a change in the physical properties of the fuel, or the fuel/air mixture, e.g., the surface tension of the fuel, resulting in improved mixing and reduced soot and smoke formation.

The reference above to the presence of the dispersant includes the presence of a reaction product of the dispersant with a component of the fuel, the reaction having taken place either before entry into the combustion chamber or within the combustion chamber prior to combustion.

The alkyl nitrate used according to the invention may contain, as indicated above, from 13 to 25 carbon atoms. Advantageously, the alkyl group contains from 13 to 20, preferably from 13 to 18 carbon atoms. It is within the scope of the invention to use mixtures of two or more alkyl nitrates with carbon numbers within the

specified range, to use two or more different isomers of a given carbon number, and to use one or more different isomers of a given carbon number in admixture with one or more isomers of a different given carbon number, within the specified range. It is also within the scope of the invention to employ one or more alkyl nitrates with carbon numbers within the specified range with other cetane enhancers, especially aliphatic, more especially alkyl, nitrates with carbon numbers outside the specified range, more especially with from 9 to 12 carbon atoms, or with aliphatic nitrates other than alkyl nitrates with carbon numbers within the specified range.

Advantageously, the alkyl nitrates of the invention are obtained by nitration of oxo process alcohols, which normally contain a mixture of isomers of the nominal alcohol, together with alcohols of slightly higher and lower carbon numbers. By the use of different isomers of the same alkyl groups, optionally in admixture with isomers of alkyl groups with, for example up to 3, carbon atoms more or less than the nominal alkyl group, a nitrate admixture with a range of boiling points may be obtained. By matching this range with the boiling range of the fuel, combustion may be enhanced and particulate emissions reduced.

An especially preferred alkyl nitrate is one derived from the nitration of a C ₁₃ oxo process alcohol, especially one obtained by hydroformylation of the

reaction product of tetramerization of propene. A product mixture having a lower proportion of highly branched isomers is preferred to one having a greater proportion.

The alkyl nitrate is advantageously used in the fuel at a concentration of from 0.005 to 5.0 %, preferably 0.01 to 2 %, and more preferably from 0.1 to 1.0 %, by weight, based on the weight of the fuel.

The ashless dispersant is advantageously one obtainable by the reaction of a hydrocarbon containing, for example, 30 to 500 carbon atoms, with an unsaturated acid or anhydride, and the further reaction of the resulting product with a polyamine. A typical such product is a polyisobutylene succinic acid polyamine.

More especially, however, the dispersant is one containing the group -N=C-N-C=0, of which the -N=C-N group forms part of a ring, and of which the carbon and nitrogen atoms of the C-N=C=0 group form part of a different ring.

Preferably, the ashless dispersant is an oil soluble compound of the formula:

R ⁶ R ⁴ NR ⁵ [NR ⁴ R ⁵ ] _b NR ⁴ R ⁴ (II)

or mixtures of two or more such compounds, wherein R ¹ , R ² and R ³ may be the same or different and are independently hydrogen or a hydrocarbyl substituent having from 2 to 600 carbon atoms, or a keto, halo, hydroxy, nitro, cyano, or alkoxy derivative thereof, provided that at least one of R ¹ , R ² and R ³ is a hydrocarbyl substituent having from 2 to 600 carbon atoms or said derivative thereof, or wherein R ¹ and R ² together form a hydrocarbylene sub¬ stituent having 4 to 600 carbon atoms or a keto, halo, hydroxy, nitro, cyano or alkoxy derivative thereof, provided that R ¹ and R ² together with the carbon atom which forms the C-R ¹ bond with R ¹ and the nitrogen atom which forms the N-R ² bond with R ² form a ring having at least 5 members, wherein Z represents -R^CNR ¹¹ ^ ¹⁰ )^- or

-[R ¹⁰ R ^1:L N] _d R ¹⁰ [NR ^1:L R ¹⁰ ] _e wherein each R ¹⁰ , which may be the same or different, represents an alkylene group having from 1 to 5 carbon atoms in its chain, each R ¹¹ , which may be the same or different, represents a hydrogen atom or a hydrocarbyl group, and c is from 0 to 6, d is from 1 to 4, e is from 1 to 4, provided that d + e is at most 5, each R ⁴ is independently H or an alkyl group having up to 12 carbon atoms, R ⁵ is an alkylene group having up to 6 carbon atoms in the chain, optionally substituted by one or more hydrocarbyl groups having up to 10 carbon atoms, an acyl

group having from 2 to 10 carbon atoms, or a keto, halo, hydroxy, nitro, cyano or alkoxy derivative of a hydrocarbyl group having from 1 to 10 carbon atoms or of an acyl group having from 2 to 10 carbon atoms, R ⁶ is a hydrocarbyl substituent having from 2 to 600 carbon atoms or said derivative thereof, a is from 1 to 150, and b is from 0 to 12. Advantageously, when c is zero, there are at least two carbon atoms in the R ¹⁰ alkylene chain, and preferably there are three.

As used in this specification the term "hydrocarbyl" refers to a group having a carbon atom directly attached to the rest of the molecule and having a hydrocarbon or predominantly hydrocarbon character. Among these, there may be mentioned hydrocarbon groups, including aliphatic, (e.g., alkyl or alkenyl) , alicyclic (e.g., cycloalkyl or cycloalkenyl) , aromatic, aliphatic- and alicyclic-substituted aromatic, and aromatic- substituted aliphatic and alicyclic groups. Aliphatic groups are advantageously saturated. Examples include methyl, ethyl, propyl, butyl, isobutyl, pentyl, hexyl, octyl, decyl, octadecyl, cyclohexyl, and phenyl. These groups may, as indicated above, contain non-hydrocarbon substituents provided they do not alter the predominantly hydrocarbon character of the group. Examples include keto, halo, hydroxy, nitro, cyano, alkoxy and acyl. If the hydrocarbyl group is substituted, a single (mono) substituent is preferred. Examples of substituted

hydrocarbyl groups include 2-hydroxyethyl, 3- hydroxypropyl, 4-hydroxybutyl, 2-ketopropyl, ethoxyethyl, and propoxypropyl. The groups may also or alternatively contain atoms other than carbon in a chain or ring otherwise composed of carbon atoms. Suitable hetero atoms include, for example, nitrogen, oxygen and sulfur. The term "hydrocarbylene" is used analogously; such a group is attached to the rest of the molecule at least at one end and preferably at both ends through a carbon atom.

Advantageously, there is used a compound of the formula:

IV

wherein R ⁷ is hydrogen or a hydrocarbyl substituent having from 1 to 600 carbon atoms, R ⁸ is hydrogen or a C to C ₁₂ hydrocarbyl substituent and, if there is more than one R ⁸ in a compound, they may be the same or different, R ⁹ is a hydrocarbylene substituent having from 2 to 600 carbon atoms, two of which carbon atoms are bonded to the α-carbon atoms of the succinic anhydride based ring, X^ represents hydrogen or an alkyl group having from 1 to 12 carbon atoms, X ₂ represents hydrogen, an alkyl group having from 1 to 12 carbon atoms, a hydroxy group, or an alkoxy group, having from 1 to 12 carbon atoms, or X and X2 may together represent an oxygen or sulphur atom, Z has the meaning given above, and h is from 1 to 20. Advantageously h represents 1.

Although in R ¹⁰ the alkylene chain may have at most 5 carbon atoms, it may be branched, and the length of the branch or branches is not limited. When R ³ represents a hydrocarbyl substituent, and Z contains a nitrogen atom, the hydrocarbyl substituent is advantageously linked to

the, or a, nitrogen atom. The nitrogen-hydrocarbyl linkage may in that case be, e.g., an amide linkage.

Pref rably there is used a macrocyclic polyamine compound of the formula:

or

or mixtures of two or more such compounds, wherein R ¹² is a hydrocarbyl substituent having from 2 to 400 carbon atoms, R ¹³ is hydrogen or a C^ to C ₁₂ hydrocarbyl substituent and, if there is more than one R ¹³ in a compound they may the same or different, R ¹⁴ is a hydrocarbylene substituent having from 4 to 400 carbon atoms, two of which carbon atoms are bonded to the α- carbon atoms of the succinic anhydride based ring, and -Z- represents -CH ₂ CH ₂ CH ₂ -; -(CH ₂ CH ₂ CH2NH) _n CH2CH ₂ CH2-, where n is 1 to 6, or

-(CH ₂ CH2CH2NH) _m (CH2)p(NHCH2CH ₂ CH2)q- where m and q are each at least 1 and m+q = 2 to 5, p is 1 to 5, advantageously 2 to 5, and a is 1 to 20.

A method for the preparation of the macrocyclic polyamines of the formulae above is described, for example, in U.S. Patent No. 4 637 886, the disclosure of which is incorporated by reference herein. Formation of the macrocyclic and optionally polymacrocyclic compounds proceeds by the aminolysis of hydrocarbyl succinic anhydride, monocarboxylic acid or polycarboxylic acid, adding the acid or anhydride to the di or polyamide compound, as described in more detail in the above- referenced U.S. Patent.

The hydrocarbyl and hydrocarbylene substituents R ¹² and R ¹⁴ are advantageously derived from a polymer based on a major amount of a C ₂ to C ₅ olefin, for example homo or copolymers of ethylene, propylene, butylene (1 or 2) ,

pentylene and, especially, isobutylene. Polyisobutylene is especially preferred. When the polymer is a copolymer, it may be a copolymer of two or more of the specified monomers, or a copolymer of one or more of the specified monomers with a copolymerizable unsaturated monomer; when the polymer is a copolymer it may be a block or a random copolymer.

The polymer advantageously has from 5 to 300 carbon atoms, preferably 10 to 200 carbon atoms and most preferably 20 to 100 carbon atoms. Preparation of the alkyl and alkenyl succinic anhydrides which form convenient reactants for the cyclodehydration reaction by which the macrocyclic polyamine may be produced is described, for example, in U.S. Patents Nos. 3 018 250 and 3 024 195, the disclosures of which are incorporated by reference herein.

Suitable amine reactants are of the formula H2-Z- H2, where Z has the meaning given above. Preferred amines include 1,3-propane diamine; 3,3'-imino- bis-propylamin , ,N ^l -bis(3-aminopropyl)ethylene diamine; N, '-bis(3-aminopropyl)-1,3-propane diamine; other suitable amines include ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pen amine, pentamethylene hexamine, dipropylene triamine, tripropylene tetramine, tetrapropylene pentamine and pentapropylene hexamine.

The mole ratio of alkenyl or alkyl succinic

anhydride to polyamine used in the preferred preparation of the macrocyclic polyamines may vary, for example, from 0.2:1 to 5:1, and is preferably from 0.5:1 to 2:1, more preferably from 0.5:1 to 1.5:1, and most preferably from 0.5:1 to 1:1.

As monocarboxylic acid there may be used an acid of the formula:

R ¹⁵ -C00H where R- ¹ - ⁵ is hydrogen, or a substituted or unsubstituted alkyl, cycloalkyl, alkenyl, cycloalkenyl, or aryl group. Examples of such acids include formic acid, acetic acid, propionic acid, butyric acid, valeric acid, palmitic acid, stearic acid, cyclohexanecarboxylic acid, 2- methylcyclohexane carboxylic acid, 4-methylcyclohexane carboxylic acid, oleic acid, linoleic acid, linolenic acid, cyclohex-2-eneoic acid, benzoic acid, 2-methylbenzoic acid, 3-methylbenzoic acid, 4-methyl- benzoic acid, salicylic acid, 2-hydroxy-4-methylbenzoic acid, 2-hydroxy-4-ethylsalicylic acid, p-hydroxybenzoic acid, 3,5-di-tert-butyl-4-hydroxybenzoic acid, o-aminobenzoic acid, p-aminobenzoic acid, o-methoxybenzoic acid and p-methoxybenzoic acid.

As dicarboxylic acid there may be used an acid of the formula:

H ^00C - ^(C H ₂ )t- ^C00H where t is zero or an integer, including e.g. oxalic acid, malonic acid, succinic acid, glutaric acid, adipic

acid, pimelic acid and suberic acid, or an acid of the formula:

Rl5 HOOC-(CH ₂ ) _X -CH-(CH ₂ ) _y -C00H where t is zero or an integer, y is zero or an integer and x and y may be equal or different and R ¹⁵ is as defined above. Examples of such acids include the alkyl or alkenyl succinic acids, 2-methylbutanedioic acid, 2-ethylpentanedioic acid, 2-n-dodecylbutanedioic acid, 2-n-dodecenylbutanedioic acid, 2-phenylbutanedioic acid, and 2-(p-methylphenyl)butanedioic acid. Also included are polysubstituted alkyl dicarboxylic acids wherein other R ¹⁵ groups as described above may be substituted on the alkyl chain. These other groups may be substituted on the same carbon atom or different atoms. Such examples include 2,2-dimethylbutanedioic acid; 2,3-dimethylbutanedioic aci ; 2,3,4-trimethylpentanedioic acid; 2,2,3-trimethylpentanedioic acid; and 2-ethyl-3- methylbutanedioic acid.

The dicarboxylic acids also include acids of the formula:

HOOC-(C _r H _2r _2) ^C00H where r is an integer of 2 or more. Examples include aleic acid, fumaric acid, pent-2-enedioic acid, hex-2- enedioic acid; hex-3-enedioic acid, 5-methylhex-2- enedioic acid; 2,3-di-methylpent-2-enedioic acid;

2-methylbut-2-enedioic acid; 2-dodecylbut-2-enedioic acid; and 2-polyisobutylbut-2-enedioic acid.

The dicarboxylic acids also include aromatic dicarboxylic acids e.g. phthalic acid, isophthalic acid, terephthalic acid and substituted phthalic acids of the formula:

where R ¹⁵ is as defined above and n = 1, 2, 3 or 4 and when n > 1 then the R groups may be the same or different. Examples of such acids include 3-methyl- benzene-1,2-dicarboxylic acid; 4-phenylbenzene-l,3- dicarboxylic acid; 2-(l-propenyl)benzene-l,4-dicar- boxylic acid, and 3,4-dimethylbenzene-l,2-dicarboxylic acid.

Advantageously the compound of the formula II is a compound of the formula:

R ¹² R ¹³ N-(CR ¹⁶ R ¹⁷ ) _n -[NH(CR ¹⁸ R ¹⁹ ) _u ] _b -NR ¹³ R ¹³ in which in the or each such compound the R ¹³ s may be the same or different, and in which R ¹² and R ¹³ have the meanings given above, R ¹⁶ , R ¹⁷ , R ¹⁸ , and R ¹⁹ are independently hydrogen, a hydrocarbyl group having from 1 to 10 carbon atoms, an acyl group having from 2 to 10 carbon atoms, or a monoketo, monohydroxy, mononitro, monocyano or alkoxy derivative of a hydrocarbyl group

having from 1 to 10 carbon atoms or of an acyl group having from 2 to 10 carbon atoms, n is from 1 to 6, u is from 1 to 6, and b is from 0 to 12.

The preparation of such compounds is described for example in U.S. Patents Nos. 3 438 757, 3 565 804, 3 -574 576, 3 671511, 3 898 056, and 3 960 515, and British Patents No. 1 254 338 and 1 398 067, the disclosures of which are incorporated herein by reference. In this embodiment, the preferred hydrocarbyl substituents represented by R ¹² are as given above with reference to the macrocyclic polyamines.

The polyamine used to derive the hydrocarbyl polyamine is advantageously a compound having from 2 to 12 nitrogen atoms and from 2 to 40 carbon atoms. The preferred hydrocarbyl polyamines for use in this invention are compounds derived from polyalkylene polyamines, including alkylene diamines and substituted polyalkylene polyamines. Preferably, the alkylene group contains from 2 to 6 carbon atoms, there being preferably from 2 to 3 carbon atoms between the nitrogen atoms. Examples of such polyamines include ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, di(trimethylene)triamine, propylene diamine, dipropylene triamine, tripropylene tetramine, N-methyl ethylene diamine, N-N,-dimethyl ethylene diamine, N- methyl-l,3-diamino propane and N,N-dimethyl-l,3-diamino propane. Such amines include branched chain polyamines

and cyclic structures formed by reaction of linear polyamines. Among the polyalkylene polyamines those containing from 2 to 12 nitrogen atoms and from 2 to 24 carbon atoms are especially preferred.

The mole ratios of alkyl or alkenyl halide to polyamine, used in accordance with the preferred method of making the compounds, are as described above for the succinic anhydride/polyamine reaction.

In further embodiments of this invention, the ashless dispersant used may be a derivative of the macrocyclic or hydrocarbyl polyamines described above, such derivative being one obtainable by, and preferably one obtained by, post-treatment with, for example, boron oxide, boron oxide hydrate, a boron halide, a boron acid, sulphur, a sulphur chloride, a phosphorus oxide or sulphide, a carboxylic acid or anhydride, an acyl halide, an epoxide, an episulphide or acrylonitrile. Methods for carrying out such treatment are well known in the art; for example boration to incorporate 0.1 to 1 atoms of boron for each nitrogen atom may be carried out as described in U.S. Patent No. 3 254 025, the disclosure of which is incorporated by reference herein.

A preferred post-treatment for the formation of an additive is treatment with polyisobutylene succinic anhydride. Advantageously, the macrocyclic or hydrocarbyl polyamine is treated with 10 to 50 mole % of an anhydride formed from a polyisobutylene of molecular

weight 900 to 1200, for example by reaction at 120°C for an hour or until the reaction mixture contains no free anhydride.

Advantageously the concentration of the ashless dispersant in the fuel is in the range of from 0.0005 to 2, preferably from 0.001 to 0.5, and more preferably from 0.005 to 0.3%, by weight, based on the weight of the fuel.

Among foam inhibitors there may be mentioned siloxane-polyoxyalkylene copolymers, for example those described in U.S. Patent No. 3 233 986, the disclosure of which is incorporated by reference herein, which comprise at least one siloxane block containing at least two siloxane groups of the formula R2 ^s i°o.5(4-b) wherein R represents a halogen atom or an optionally halogenated hydrocarbon group and b represents from 1 to 3, and at least one polyoxyalkylene block containing at least two oxyalkylene groups. Generally, the alkylene groups have 2 or 3 carbon atoms, and usually both ethyleneoxy and propyleneoxy groups are present. Advantageously, the copolymer is a polymethylsiloxane-polyoxyalkylene copolymer, preferably of the general formula

(CH ₃ ) ₃ SiO[CH ₃ (A)SiO] _m [(CH ₃ ) ₂ SiO] _n Si(CH ₃ ) ₃ in which A represents

-(CH ₂ )pO(C ₂ H4θ) _χ (C ₃ H ₆ 0) _y Z in which Z represents hydrocarbyl, OC(hydrocarbyl) or, preferably, hydrogen, and in which the absolute values of

m and n, and their ratios, and the values of p, x, and y, and their ratios, may vary widely but total values advantageously give a weight average molecular weight in the range of from 600 to 25000. The ratio of m:n is advantageously in the range of from 10:1 to 1:20, or the value of n may be zero, and the ratio of x:y is advantageously in the range of from 1:100 to 100:1, preferably 20:80 to 100:1, or one of x or y, but not both, may be zero. Preferred foam inhibitors are those sold under the trade mark TEGOPREN by Th. Goldschmidt AG. Advantageously, the foam inhibitor is present in the fuel in a proportion in the range of from 0.0001 to 0.2%, preferably from 0.005 to 0.02%, by weight.

The additive concentrate of the invention advantageously comprises the C ₁₃ to C ₂ 5 alkyl nitrate and the ashless dispersant in admixture with a fuel oil, or in a solvent miscible with a fuel oil, advantageously in a ratio in the range of from 5:95 to 95:5 parts by weight of total additive:oil or solvent, preferably in a weight ratio from 30:70 to 70:30. The alkyl nitrate: dispersant ratio is advantageously from 50:1 to 1:2, and preferably from 20:1 to 1:1.

Advantageously, the additive concentrate also comprises a foam inhibitor. In such concentrate, the alkyl nitrate:foam inhibitor ratio is advantageously from 500:1 to 2:1.

The use of other additives does not adversely affect

the performance of the alkyl nitrate. In some cases the use of another additive or additives may lead to a reduction in emissions greater than might be expected. Other additives which may be used include, for example, diesel detergents, antifoam additives, antirust additives, and de ulsifiers. These other additives may be present in the fuel in a total concentration of 0.001 to 1, preferably 0.005 to 0.2, and most preferably a total concentration of 0.005 to 0.15%, based on the total weight of fuel.

The following examples, in which all parts and percentages are by weight unless otherwise indicated, illustrate the invention.

Example 1 A cetane improver according to the invention and a commercial octyl nitrate were tested in an engine to determine their effect on particulate emissions. The engine used was a 6 cylinder 4 stroke naturally aspirated DI engine with the following specification: Swept Volume = 5958cc. Maximum Power = 100 KW at 2800rpm. Maximum Torque = 402 NM at 1400rpm. Compression Ratio = 17.25:1

The fuel used in the tests was a standard UK automotive diesel fuel. A typical analysis was:

Specific Gravity = 0.849 kg/litre Cetane Index = 51.5 Distillation °C IBP 162

20% 252

50% 286

90% 338

FBP 369

Sulphur Content = 0.23% Flash Point = 69°C The additives were compared at 500ppm using tests run in the following manner:

1. The engine was conditioned prior to testing each additive by running the engine on the test fuel at 75% speed and 75% load for 12 hours.

2. The emissions from the engine were then measured using a standard ECE R49 thirteen mode test.

The test consists of thirteen modes, i.e., engine speed and load combinations, each of 6 minutes duration, run consecutively over a total test time of 78 minutes, emission measurements being made in each mode. The results, obtained from the individual mode results following the procedure laid down in the R49 test, are shown in Table 1.

In the first test in Table 1, the fuel contained no

cetane enhancer. In the test marked "comparison" a commercial octyl nitrate was used. In the test marked Example 1 an oxo process C^ ₃ alcohol nitrate was employed, the product having been obtained by nitration of the alkanol obtained by hydroformylating the dodecene resulting from tetramerizing propylene.

Table 1 Additive Particulate (q/kWhr) nil 0.880

Comparison 0.732

Example 1 0.690

In the test using the fuel with no additive, the running of the engine for 12 hours in stage 1 ensures the formation of injector and other fuel-related deposits for the test in stage 2 and allows the particulate emission level to reach a stable level. In the tests of the fuels containing an additive, stage 1 removes any pre-existing deposits, or maintains the engine in a clean state, for the test in stage 2. The results clearly show that with the same mass of additive, and a much reduced mass of the nitrate group, a reduction in particulate emissions is achieved using the C ₁₃ additive according to the invention comparable to that achieved using the commer¬ cial C ₈ additive.

Examples 2 and 3 The engine of Example 1 was used to compare the improvement in cetane number achieved by using two alkyl nitrates according to the invention with that obtained using the octyl nitrate (comparison) as in Example 1. Two fuels were used. Fuel A (the fuel used in Example 1) had a cetane rating of 51.5, Fuel B had a rating of 47.1, each value being the average of 8 tests. The alkyl nitrates of Examples 2 and 3 were both C ₁₃ alkyl nitrates obtained by tetramerizing propylene, hydroformylating the resulting dodecene, and nitrating the resulting alcohol. The alkyl chain of the nitrate of Example 2 (the same material as used in Example 1) was less highly branched than that of Example 3. The additives were each tested on fuels A and B at treat rates of 500 ppm and 1000 ppm. The results are shown in Table 2, in terms of the increase in cetane ratings over those of the two base fuels.

Table 2 Fuel A Fuel B

The Table shows that at 500 ppm the improvement in cetane rating is comparable with that using octyl nitrate, especially when using the less highly branched material of Example 2. At the 1000 ppm level, although the improvement is less than that using octyl nitrate, it is still significant.