Cerium oxide is widely used as a catalyst in three way converters for the reduction of toxic exhaust emission gases and the reduction in particulate emission in automobiles. The ceria contained within the catalyst can act as a chemically active component, working as an oxygen store by release of oxygen in the presence of reductive gases, and removal of oxygen by interaction with oxidising species.

Cerium oxide may store and release oxygen by the following processes:- 2CeOZ HCe203 + 1/202 The key to the use of ceria for catalytic purposes is the low redox potential between the Ce3+ and Ce4+ ions (1. 7V) that allows the above reaction to easily occur in exhaust gases. Cerium oxide may provide oxygen for the oxidation of CO or CnHn or may absorb oxygen for the reduction of NO.. The amounts of oxygen reversibly provided in and removed from the gas phase are called the oxygen storage capacity (OSC) of ceria.

The above catalytic activity may occur when cerium oxide is added as an additive to fuel. However, in order for this effect to be useful the cerium oxide must be of a particle size small enough to remain in a stable dispersion in the fuel. The cerium oxide particles must be of a nanocrystalline nature, for example they should be less than 1 micron in size, and preferentially 1-300nm in size. In addition, as catalytic effects are surface area dependant the small particle size renders the nanocrystalline material more effective as a catalyst.

The incorporation of cerium oxide in fuel serves more than one purpose. The primary purpose is to act as a catalyst in the reduction of toxic exhaust gases on combustion of the fuel. However, it can serve another purpose in diesel engines.

Diesel engines increasingly comprise a trap for particulates resulting from combustion of the diesel fuel. The presence of the cerium oxide in the traps helps to burn off the particulates which accumulate in the trap. Indeed such a use is in commercial operation. Thus certain vehicles, principally those devised by Peugeot, incorporate an on-board dosing system whereby a cerium based additive is incorporated into the fuel before the latter enters the engine. This is, though, a complicated system. Effectively, the on-board system enables the particulate filter to be regenerated so that it lasts much longer.

The applicants have now found, according to the present invention, that there is a particular advantage of incorporating cerium oxide nanoparticles into a fuel for an internal combustion engine which comprises a lubricant (lubricating oil). This can provide for better control of the amount of ceria used. The present invention has applicability particularly in relation to two-stroke engines which are fuelled by a mixture of gasoline and lubricating oil ; such engines are particularly useful in motor cycles and snowmobiles, outboards and portable power equipment such as chain saws. In particular, by adding the cerium oxide to the lubricating oil which forms only a small proportion of the fuel it is possible to dose the cerium oxide particles more accurately than if they are added to the final fuel.

Accordingly, the present invention provides a method of improving the efficiency of a fuel for an internal combustion engine, which fuel comprises gasoline and a lubricating oil, which comprises adding to the fuel or the oil cerium oxide and/or doped cerium oxide. The present invention also provides a lubricating oil composition which comprises, apart from the lubricating oil, cerium oxide and/or doped cerium oxide as well as a fuel comprising such a lubricating oil composition and gasoline.

Although it is possible to use ordinary cerium oxide particles it has been found to be beneficial to use cerium oxide which has been doped with components that result in additional oxygen vacancies being formed. This generally means that the dopant will be di-or tri-valent in order to provide oxygen vacancies.

Such dopant ions must be di-or tri-valent ions of an element which is a rare earth metal, a transition metal or a metal of Group IIA, BIB, VB, or VIB of the Periodic Table in order to provide oxygen vacancies. They must also be of a size that allows incorporation of the ion within the surface region of the cerium oxide nanoparticles. Accordingly metals with a large ionic radius should not be used. For example transition metals in the first and second row of transition metals are generally preferred over those listed in the third. The ceria serves as the oxygen activation and exchange medium during a redox reaction. However, because ceria and the like are ceramic materials, they have low electronic conductivity and low activity surface sites for the chemisorption of the reacting species. Transition metal additives are particularly useful to improve this situation. In addition, multivalent dopants will also have a catalytic effect of their own.

Typically the doped oxides will have the formula Cel xMxO2 where M is a said metal or metalloid, in particular Rh Cu, Ag, Au, Pd, Pt, Sb, Se, Fe, Ga, Mg, Mn, Cr, Be, B, Co, V and Ca as well as Pr, Sm and Gd and x has a value up to 0.3, typically 0. 01 or 0.1 to 0.2, or of the formula [ (Ce02) i-n (REOy) n] i-kM'k where M'is a said metal or metalloid other than a rare earth, RE is a rare earth, y is 1 or 1.5 and each of n and k, which may be the same or different, has a value op to 0.5, preferably up to 0.3, typically 0. 01 or 0. 1 to 0.2.

Copper is particularly preferred. Further details can be found in our PCT Application GB2002/005013 to which reference should be made.

In general the cerium oxide particles will have a size not exceeding 1 micron and especially not exceeding 300 nm, for example 1 to 300 nm, such as from 1 to 150 nm, in particular 2 to 50 nm, especially 2 to 20 nm.

It is preferred that the particles are coated to prevent agglomeration. For this purpose the particles can be comminuted in an organic solvent in the presence of a coating agent which is an organic acid, anhydride or ester or a Lewis base. It has been found that, in this way which involves coating in situ, it is possible to significantly improve the coating of the oxide. Further, the resulting product can, in many instances, be used directly without any intermediate step. Thus in some coating procedures it is necessary to dry the coated particles before dispersing them in a hydrocarbon solvent.

Thus the cerium oxide can be dispersible or soluble in the (liquid) fuel or another hydrocarbon compatible with the fuel.

The particles which are subjected to the process should have as large a surface area as possible and preferably the particles have a surface area, before coating, of at least 10 m2/g and preferably a surface area of at least 50 or 75 m2/g, for example 80- 150 m2/g, or 100-300m2/g.

The coating agent is suitably an organic acid, anhydride or ester or a Lewis base. The coating agent is preferably an organic carboxylic acid or an anhydride, typically one possessing at least 8 carbon atoms, for example 10 to 25 carbon atoms, especially 12 to 18 carbon atoms such as stearic acid, It will be appreciated that the carbon chain can be saturated or unsaturated, for example ethylenically unsaturated as in oleic acid. Similar comments apply to the anhydrides which can be used. A preferred anhydride is dodecenylsuccinic anhydride. Other organic acids, anhydrides and esters which can be used in the process of the present invention include those derived from phosphoric acid and sulphonic acid. The esters are typically aliphatic esters, for example alkyl esters where both the acid and ester parts have 4 to 18 carbon atoms.

The coating process can be carried out in an organic solvent. Preferably, the solvent is non-polar and is also preferably non-hydrophilic. It can be an aliphatic or an aromatic solvent. Typical examples include toluene, xylene, petrols diesel fuel as . well as heavier fuel oils. Naturally, the organic solvent used should be selected so that it is compatible with the intended end use of the coated particles. The presence of water should be avoided ; the use of an anhydride as coating agent helps to eliminate any water present.

The coating process involves comminuting the particles so as to prevent any agglomerates from forming. Techniques which can be used for this purpose include high-speed stirring or tumbling and the use of a colloid mill, ultrasonics or ball milling. Ball milling is preferred. Further details of such coatings can be found in PCT/GB02/02312.

In general it has been found that the cerium oxide particles can be stabilised in the lubricating formulation by the presence of a dispersant. Numerous other additives can also be incorporated. Typical formulations of lubricating oil are given below: Typical concentration preferable concentration Dispersant 1-15% 1-10% Detergent 0-5% up to 3% Smoke suppressant 0-30% up to 20% Antioxidants 0-2% up to 2% Corrosion Inhibitor 0-2% up to 2% Friction Modifier 0-5% up to 3% Diluent 0-30% up to 25% Base Oil 10-95% 20-60% Suitable dispersants generally possess a polar group attached to a relatively high molecular weight hydrocarbon chain. Typical dispersants include N-substituted long chain aliphatic succinimides which may be saturated or unsaturated, especially ethylenically unsaturated, in particular those of structure: where each Ri is independently an alkyl group, especially a polyisobutyl group, typically with a molecular weight of 500-5000, and R2, independently, alkenyl groups, commonly ethylenyl (C2H4) groups. Such molecules are commonly derived from reaction of an alkenyl acylating agent with a polyamine, and a wide variety of linkages between the two moieties is possible beside the simple imide structure shown above, including a variety of amides and quaternary ammonium salts.

Suitable materials have a polyisobutylene molecular weight typically from 750 to 2500, especially 900 to 1500 with those having molecular weights around 950 and 1300 being particularly useful, although an aliphatic succinimide with a molecular weight of about 2100 is also useful. Specific succinimides include polyisobutenyl succinimides. Succinimide dispersants are more fully described in U. S. Patent 4,234, 435. Related to these are high molecular weight esters. These materials are similar to the above-described succinimides except that they may be seen as having been prepared by reaction of a hydrocarbyl acylating agent and a polyhydric aliphatic alcohol such as glycerol, pentaerythritol, or sorbitol. Such materials are described in more detail in U.-S. Patent 3, 381, 022.

Other suitable dispersants are Mannich bases. These are materials which are formed by the condensation of a higher molecular weight, alkyl substituted phenol, n alkylene polyamine, and an aldehyde such as formaldehyde. Such materials typically have the general structure (including a variety of isomers and the like) and are described in more detail in U. S.

Patent 3,634, 515.

Other dispersants include alkyl phenols, such as those of the formula (R) a-Ar- (OH) b. As used herein, the term"phenol"is used in its art-accepted generic sense to refer to hydroxy-aromatic compounds having at least one hydroxyl group bonded directly to a carbon of an aromatic ring. In the formula, Ar represent an aromatic moiety, which can be a single aromatic nucleus such as a benzene nucleus, which is preferred, a pyridine nucleus, a thiophene nucleus or a 1,2, 3,4-tetrahydronaphthalene nucleus, or a polynuclear aromatic moiety. Polynuclear moieties can be of the fused type, such as found in naphthalene and anthracene, or of the linked type, wherein at least two nuclei are linked through bridging linkages to each other.

In the above formula, R represents a substantially saturated hydrocarbyl group, preferably containing at least about 10 aliphatic carbon atoms. More than one such group can be present, but usually no more than 2 or 3 such groups are present for each aromatic nucleus in the aromatic moiety Ar. Usually each R contains at least 30, more typically at least 50 aliphatic carbon atoms, typically up to 400, more commonly up to 300. Other substituent groups may likewise be present, including, in particular, one or more amino groups.

Certain amines can also be employed as dispersants. Examples of useful amino compounds include aliphatic, eycloaliphalic, or heterocyclic amines and polyamines, and mixtures thereof. Polyamines are preferred. Aliphatic monoamines can be primary, secondary, or tertiary. Hydroxyamines can also be employed.

Among aliphatic polyamines are alkylene polyamines including those having the formula R1N Rl (U-NR*) nRl where U is an alkylene group of 2 to 10 carbon atoms, each Rl is independently a hydrogen atom, a lower alkyl group, a lower hydroxyalkyl group, or a lower aminoalkyl group (provided that at least one Rl is a hydrogen atom) and n is 1 to 10. Specific examples include methylene polyamine, ethylene polyamines, propylene polyamine, and butylene polyamines, including ethylene diamine, diethylene triamine, triethylene tetramine, and higher homologues, such as the polyalkylene polyamines (e. g., Jeffamine).

Additional dispersants include the product of the reaction of a fatty monocarboxylic acid of 12-30 carbon atoms and one or more of the afore-described alkylene amines. The fatty monocarboxylic acids are generally mixtures of straight and branched chain fatty carboxylic acid contains 12 to 30 carbon atoms. Branched chain fatty carboxylic acid/alkylene polyamine products have been described extensively in the art ; in this regard, reference can be made to U. S. Patent numbers 3,110, 673 and 3,857, 791.

Other dispersants include polymeric dispersant additives, which are generally hydrocarbon-based polymers which contain polar functionality to impart dispersancy characteristics to the polymer. Included in this category are interpolymers of oil- solubilizing monomers such as decyl methacrylate, vinyl decyl ether, and high molecular weight olefins, with monomers containing polar substituents, e. g., aminoalkyl acrylates or acrylamides and poly- (oxyethylene)-substituted. acrylates.

Suitable detergents are certain neutral or basic metal salts, including overbased materials. Overbased materials, otherwise referred to as overbased or superbased salts, are generally single phase, homogeneous Newtonian systems characterized by a metal content in excess of that which would be present for neutralization according to the stoichiometry of the metal and the particular acidic organic compound reacted with the metal. In many cases overbased materials are prepared by reacting an acidic material, typically an inorganic acid or lower carboxylic acid, preferably carbon dioxide, in which case the materials are referred to as"carbonated", with a mixture comprising an acidic organic compound, a reaction medium comprising at least one inert organic solvent (e. g. mineral oil, naphtha, toluene or xylene) for said acidic organic material, a stoichiometric excess of a metal base, and a promoter such as a phenol or alcohol. The acidic organic material will normally have a sufficient number of carbon atoms to provide a degree of solubility in oil. The acidic organic material can be, for example, as carboxylic acid, sulfonic acid, phosphoric acid, phenol, or a multifunctional material such as a salicylate or the condensate of an alkyl phenol and glyoxylic acid, and will normally have at least one hydrocarbyl substituent.

The amount of excess metal in an overbased material is commonly expressed in terms of"metal ratio"which is the ratio of the total equivalents of the metal to the equivalents of the acidic organic compound. A neutral metal salt has a metal ratio of one. A salt having 4.5 times as much metal as present in a normal salt will have metal excess of 3.5 equivalents, or a ratio of 4.5.

Such overbased materials are well known to those skilled in the art.

Reference can be made to the basic sails of sulfonic acids, carboxylic acids, phenols, phosphoric acids. and mixtures of any two or more of these discussed in U. S. Patents 2,501, 731 ; 2,616, 905; 2,616, 911; 2,616, 925; 2,777, 874; 3,256, 186., 3,384, 585; 3,365, 396 ;, 320,162 ; 3,318, 809; 3,488, 284; and 3,629, 109.

Suitable smoke suppressants are thickeners. The thickener is generally a polyalkene, preferably a polyisobutene, preferably having a molecular weight of greater than 450, more preferably greater than 950. The thickener can be a reactive polyisobutene, preferably having a molecular weight of greater than 450, more preferably greater than 950. A reactive polyisobutene differs from a standard poly- isobutene in that at least 80% of the terminal unsaturation is in the alpha position.

Suitable conventional polyisobutenes are Indopol HP15 and Indopol H100 available from BP Chemicals, and suitable reactive polyisobutenes are Glycopal 550 and Glycopal 1000 available from BASF.

Suitable corrosion and oxidation-inhibiting agents (antioxidants) include chlorinated aliphatic hydrocarbons such as chlorinated wax and chlorinated aromatic compounds; organic sulfides and polysulphides; sulfurized alkylphenol ; phosphosulfurized hydrocarbons; phosphorus esters; including principally dihydrocarbon and trihydrocarbon phosphites, and metal thiocarbamates.

Many of the above-mentioned corrosion-oxidation inhibitors also serve as antiwear agents. Zinc dialkylphosphorodithioates are a well known example.

Suitable friction modifiers include fatty esters, including sorbitan and sorbitol partial carboxylic esters, such as sorbitan mono-di-and trioleates, as well as the corresponding stearate and laureate esters, or mixtures thereof; sorbitol mono, di-, and trioleates, as well as the corresponding stearate and laureate esters, or mixtures thereof; glycerol fatty esters, such as glycerol monooleate, glycerol dioleate, the corresponding mono-and di-esters from C10-C22 acids such as stearic, isostearic, behenic, and lauric acids ; corresponding mono-and diesters made from fatty acids and 2-methyl 2-hydroxymethyl-1, 3-propanediol, 2-ethyl-2-hydroxymethyl-1, 3- propanediol, and tris-hydroxymethyl-methane; the mono-, di-, and triesters from CIO- C22 fatty carboxylic acids and monopentaerythritol ; the corresponding partial fatty acid esters of di-pentaerythritol. A preferred material is glycerol monooleate.

Suitable diluents are combustible, generally hydrocarbon, solvents (other than the oil of lubricating viscosity), and in which the remaining components of the lubricant are soluble. The solvent should be combustible because it is ultimately designed to be consumed in the engine, and non-combustible character is undesirable. In order to ensure suitable combustibility, the solvent should have a flash point (ASTM D-93) of less than 105°C (220°F), preferably less than 100°C (212°F), and more preferably less than 90°C (194°F). In order to assure safety in handling, the solvent will preferably have a flash point of 32°C (90°F) or above, and preferably 60°C (140°F) or above.

The diluent should have a suitable degree of volatility. Typically therefore, its distillation characteristics (ASTM-D 86) are such that its 90% point is less than or equal to 246°C (475°F) and its dry point is less than or equal to 288°C (550°F). In preferred solvents the 90% point will be less than or equal to 232°C (450°F), and the dry point will be less than or equal to 246°C (475°F).

The diluent should be a material in which the remaining components of the lubricant composition are soluble. Ideally the remaining components will be soluble or miscible with the solvent in all proportions, but the more important consideration is that they be soluble in the concentrations in which they are employed in the actual lubricants of interest.

The diluent is preferably a hydrocarbonaceous liquid. This term is used herein in a sense analogous to that of the common, related term"hydrocarbyl". The term "hydrocarbyl substituent"or"hydrocarbyl group"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 a molecule and having predominantly hydrocarbon character.

Examples of hydrocarbyl groups include: (1) hydrocarbon substituents, that is, aliphatic (e. g., alkyl or alkenyl), alicyclic (e. g., cycloalkyl, cycloalkenyl) substituents, and aromatic-, aliphatic- and alicyclic-substituted aromatic substituents, as well as cyclic substituents wherein the ring is completed through another portion of the molecule (e. g., two substituents together form an alicyclic radical) ; (2) substituted hydrocarbon substituents, that is substituents containing non-hydrocarbon groups which do not alter the predominantly hydrocarbon substituent (e. g. , halo (especially chloro and fluoro), hydroxy, alkoxy, mercapto, alkylmercapto, nitro, nitroso, and sulfoxy) ; (3) hetero substituents, that is substituents which, while having a predominantly hydrocarbon character, contain other than carbon in a ring or chain otherwise composed of carbon atoms. Heteroatoms include sulfur, 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.

The base oil used in the present invention may be a natural or synthetic lubricating oil or a mixture thereof. Natural oils include animal oils, vegetable oils, mineral lubricating, solvent or acid treated mineral oils, and oils derived from coal or shale. Synthetic lubricating oils include hydrocarbon oils, halo-substituted hydrocarbon oils, alkylene oxide polymers, esters of dicarboxylic acids and polyols, esters of phosphorus-containing acids, polymeric tetrahydrofurans and silicon-based oils.

Specific examples of the oils of lubricating viscosity are described in US 4,326, 972,4582618 and EP 107,282 as well as D. V. Brock,"Lubricant Engineering", volume 43, pages 184-185, March, 1987.

The lubricant can be added to the fuel when it is contained within the fuel tank; it can be premixed before the fuel is added to the tank; or it can be separately metered into the fuel stream during operation of the engine. The specific amount of the lubricant to be combined with the fuel will depend on the demands of the particular engine and the characteristics of the specific lubricant. Generally the amount of the lubricant composition is 0.5 to 10 percent by weight of the fuel plus lubricant combination, preferably 1 to 4 percent by weight.

The concentration of ceria in the fuel is typically up to 1000 ppm, generally 1 to 1000 ppm and more preferably 2 to 500 ppm. Although the ceria can be added directly to the fuel, it is preferred that it is added to the lubricating oil of, typically, a two-stroke engine fuel. As the lubricant typically contains a diluent or solvent, it is possible to add the ceria in a carrier solvent which can act as the diluent or solvent ; such solvents include an aliphatic or aromatic hydrocarbon or an aliphatic alcohol.

The composition would be: 2-stroke lubricant 65.1% 2.0% w/v solution of ceria 34.9%.

This would give 200 ppm w/v in a gasoline fuel when treated at 3.07% w/w.

If the lubricant contained 30% diluent then the following formulation could be used : 2-stroke lubricant less diluent 56.7% 2.0% w/v solution of ceria 43.3% This would give 200 ppm w/v in a gasoline fuel when treated at 2.47% w/w.

Accordingly, in general the concentration of ceria in the oil additive will be 0.01 to 20%, usually 0.02 to 10% and, especially 0.1 to 5% by weight.