EP0948586A1

EP0948586A1 - Polyol ester fuels additive

Info

Publication number: EP0948586A1
Application number: EP97942478A
Authority: EP
Inventors: Elisavet P. Vrahopoulou; John E. Johnston; Richard H. Schlosberg; Simon R. Kelemen; Michael Siskin
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1996-09-13
Filing date: 1997-09-11
Publication date: 1999-10-13
Also published as: AU734585B2; BR9711783A; WO1998011177A1; CN1230210A; EP0948586A4; JP2001501231A; CA2264707A1; AU4416697A

Abstract

A polyol ester fuel additive exhibits reduced intake valve deposits and combustion chamber deposits. The ester has between about 1 % and about 35 % unconverted hydroxyl groups and is characterized as having an hydroxyl number from about 5 to about 180.

Description

POLYOL ESTER FUELS ADDITIVE

FIELD OF THE INVENTION

The present invention relates generally to a polyol ester additive for fuels applications and more particularly to a fuels additive comprising a partially esterified polyol ester which exhibits reduced engine deposits including reduced intake valve deposits ("IVD") and combustion chamber deposits ("CCD") and improves the wear and frictional performance of the materials it contacts including the crankcase oil. The polyol ester fuels additives of this invention have unconverted hydroxyl groups from the reaction product of a polyol with a branched acid, linear saturated acid or mixtures thereof.

BACKGROUND OF THE INVENTION AND DISCUSSION OF THE PRIOR ART

The formulation of fuels for internal combustion engines has become increasingly sophisticated and complex. Basic motor fuels are tailored through additives aimed to reduce carbonaceous deposits on valves and within combustion chambers, protect fuel injector orifices and other fuel delivery components such as fuel pumps, provide anti-corrosive and anti-residue benefits, and numerous other objectives.

Esters have generally excellent thermal and oxidative stability characteristics, and have been widely used in synthetic or partially synthetic crankcase lubricants. The art has recently recognized the potential role esters may serve as fuel additives. For example U.S. 5,427,591 discloses the use of certain hydroxylaromatic esters as fuels additives to reduce engine deposits. U.S. 5,21 1,721 discloses the use of polyalkylene esters as fuels additives to control engine deposits and improve fuel hazing. U.S. 5,089,028 teaches the use of a combination of a polyalkenyl succinimide, an olefinic hydrocarbon polymer, an ester and a polyether to provide engine intake valve, port fuel injector, and carburetor detergency as well as corrosion inhibitors.

These fuels additives typically rob Peter to pay Paul, in that achieving one additive objective negatively impacts other engine performance characteristics. This often results in the necessity of employing multiple additives to achieve overall enhanced performance. For example, U.S. 5,433,755 discloses multi-component additives to achieve fuel detergent benefits and anti-corrosive properties. The prior art also teaches that high molecular weight esters may survive the combustion in the cylinder and thereby be available to provide surficial lubricant benefit to the cylinder walls and piston rings while low molecular weight esters provide detergency benefits such as reduced injector deposits. U.S. 4,920,691 teaches a combination of a low molecular weight straight chain carboxylic acid ester, i.e., molecular weight less than 200, and a high molecular weight straight chain carboxylic acid ester, i.e., molecular weight ranging from 300 to 1000 to achieve both detergency benefits and cylinder wall lubrication. In addition to increasing the cost of the fuel, it has been recognized that the amount of detergent additives needs be minimized because of the deleterious effects of the by-products such additives have on crankcase lubricants; see, for example, U.S. 5,044,478. Small amounts of the by-product of these additives, upon breakdown in the combustion chamber, wind up in the crankcase lubricant and contribute to engine oil breakdown. The fuel additive of the present invention exhibits improved IVD and CCD without the lubricant debit associated with known additives. To the contrary, the formulation of the present invention improves the wear and friction performance of the crankcase oil.

SUMMARY OF THE INVENTION

The present inventors have developed a unique fuels additive which employs a polyol ester synthesized from a polyol and branched acid, saturated linear acid, or mixtures thereof, in such a manner that the resulting ester has unconverted hydroxyl groups. The resultant fuel composition displays enhanced control of intake valve deposits ("IVD"), combustion chamber deposits ("CCD"), and reduced wear and friction in both the fuel line, combustion chamber and piston/cylinder assembly. Improved IVD and CCD are achieved without the crankcase lubricant deficits normally associated with detergent additives. To the contrary, the fuel additive of this invention which may survive the combustion chamber, improves the wear and friction performance of the crankcase oil. The ester comprises the reaction product of an alcohol having the general formula R(OH)_n where R is an aliphatic group, cyclo-aliphatic group, or a combination thereof having from about 2 to 20 carbon atoms and n is at least two where the said aliphatic group is branched or linear. The ester is characterized as having a hydroxyl number ranging from greater than about 5 to about 180. The fuels referred to in this invention generally comprise distillate fuels, and typically comprise a major amount of gasoline. The ester additive comprises a minor amount of the fuel, ranging from about 10 to about 10,000 wppm. DESCRIPTION OF THE DRAWINGS

Figure 1 exhibits friction coefficients under varying load, speed and temperature conditions.

Figure 2 exhibits wear rate data under varying load, speed and temperature conditions.

DETAILED DESCRIPTION OF THE INVENTION

The fuel composition of the present invention employs a polyol ester which comprises a compound represented by the general formula R(OOCR')_n and at least one of the following compounds:

R(OOCR')_n-lOH, R(OOCR')_n.2(OH)₂, and R(OOCRV_(i)(OH)_(i)

where n is an integer having a value of at least 2, R is an aliphatic group, cycloaliphatic group, or combinations thereof containing from about 2 to about 20 or more carbon atoms where the said alphatic group is branched or linear, R' is a branched or linear hydrocarbyl group having a carbon number in the range between about C2 to C20, and (i) is an integer having a value in the range of 0 to n. Unless previously removed, the polyol ester composition may also include excess R(OH)_n.

The ester is preferably formed by reacting a polyhydroxyl compound (i.e., polyol) with at least one branched, saturated linear acid or mixtures thereof. The composition of the feed polyol and acid are adjusted so as to provide the desired composition of the product ester.

The esterification reaction is preferably conducted, with or without a catalyst, at a temperature in the range of about 140°C to about 250°C and a pressure ranging from about 30 mm Hg to 760 mm Hg for about 0.1 to 12 hours, preferably 1 to 8 hours. In a preferred embodiment, the reactor apparatus may vacuum strip acid to provide the preferred final composition. The product may then be treated in a contact process step by contacting it with a solid such as alumina, zeolite activated carbon, or clay, for example.

Alcohols

Among the alcohols which may be utilized in the reaction with the branched acid(s) and/or saturated linear acid(s) are polyhydroxyl compounds represented by the general formula:

R(OH)_n

where R is an aliphatic group, cycloaliphatic group or combinations thereof where the aliphatic group is branched or linear, and n is at least 2. The hydrocarbyl group may contain from about 2 to about 20 or more carbon atoms and is preferably an alkyl group. The hydroxyl groups may be separated by one or more carbon atoms.

The polyhydroxyl compounds generally may contain one or more oxyethylene groups and accordingly include compounds such as polyether polyols.

The following alcohols are particularly useful as polyols in the practice of the present invention: neopentyl glycol, 2,2-dimethanoI butane, trimethylol ethane, trimethylol butane, mono-pentaerythritol, technical grade pentaerythritol, di- pentaerythritol, tri-pentaerythritol, ethylene glycol, propylene glycol and poiyalkylene glycols (e.g., polyethylene glycols, polypropylene glycols, 1,4-butanediol, sorbitol and the like, 2-methylpropanediol, polybutylene glycols, etc., and blends thereof such as an oligomerized mixture of ethylene glycol and propylene glycol). The most preferred alcohols are technical grade (e.g., approximately 88% mono-, 10% di- and 1-2% tri- pentaerythritol) pentaerythritol, monopentaerythritol, di-pentaerythritol, neopentyl glycol and trimethylol propane.

Branched Acids

The branched acid is preferably a mono-carboxylic acid which has a carbon number in the range between about C to C20_> ^ore preferably about C5 to C JQ wherein methyl or ethyl branches are preferred. The mono-carboxylic acid is preferably at least one acid selected from the group consisting of: 2,2- dimethyl propionic acid (neopentanoic acid), neoheptanoic acid, neooctanoic acid, neononanoic acid, iso- hexanoic acid, neodecanoic acid, 2-ethyl hexanoic acid (2EH), 3,5,5-trimethyl hexanoic acid (TMH), isoheptanoic acid, isooctanoic acid, isononanoic acid and isodecanoic acid. One particularly preferred branched acid is 3,5,5-trimethyl hexanoic acid. The term "neo" as used herein refers to a trialkyl acetic acid, i.e., an acid which is triply substituted at the alpha carbon with alkyl groups. These alkyl groups are equal to or greater than CH3 as shown in the general structure set forth herebelow:

Rj O

I II

R₂ - C - C - OH

I ^ Alpha Carbon R3 wherein K\, R2, and R3 are greater than or equal to CH3 and not equal to hydrogen. 3,5,5-trimethyl hexanoic acid has the structure set forth herebelow:

CH3 CH₃ O

I I II

CH3-C-CH2-CH-CH2-C-OH

CH₃

Branched Oxo Acids

The branched oxo acid is preferably a mono-carboxylic oxo acid which has a carbon number in the range between about C5 to Cio, preferably C₇ ^'to Cio, wherein methyl branches are preferred. The mono-carboxylic oxo acid is at least one acid selected from the group consisting of: isopentanoic acids, isohexanoic acids, isoheptanoic acids, isooctanoic acids, isononanoic acids, and isodecanoic acids. One particularly preferred branched oxo acid is an isooctanoic acid known under the tradename Cekanoic^®8 acid, commercially available from Exxon Chemical Company. Another particularly preferred branched oxo acid is 3,5,5 trimethylhexanoic acid, a form of which is also commercially available from Exxon Chemical Company under the tradename Cekanoic^®9 or Ck^®9. The term "iso" is meant to convey a multiple isomer product made by the oxo process. It is desirable to have a branched oxo acid comprising multiple isomers, preferably more than 3 isomers, most preferably more than 5 isomers.

Branched oxo acids may be produced in the so-called "oxo" process by hydroformylation of commercial C4-C₉ olefin fractions to a corresponding branched C₅- C10 aldehyde-containing oxonation product. In the process for forming oxo acids it is desirable to form an aldehyde intermediate from the oxonation product followed by conversion of the oxo aldehyde product to an oxo acid.

In order to commercially produce oxo acids, the hydroformylation process is adjusted to maximize oxo aldehyde formation. This can be accomplished by controlling the temperature, pressure, catalyst concentration, and/or reaction time. Thereafter, the demetalled crude aldehyde product is distilled to removed oxo alcohols from the oxo aldehyde which is then oxidized according to the reaction below to produce the desired oxo acid:

RCHO + I/2O2 → RCOOH (1)

where R is a branched alkyl group.

Alternatively, oxo acids can be formed by reacting the demetalled crude aldehyde product with water in the presence of an acid-forming catalyst and in the absence of hydrogen, at a temperature in the range between about 93 to 205°C and a pressure of between about 0.1 to 6.99 Mpa, thereby converting the concentrated aldehyde-rich product to a crude acid product and separating the crude acid product into an acid-rich product and an acid-poor product.

The production of branched oxo acids from the cobalt catalyzed hydroformylation of an olefinic feedstream preferably comprises the following steps:

(a) hydroformylating an olefinic feedstream by reaction with carbon monoxide and hydrogen (i.e., synthesis gas) in the presence of a hydroformylation catalyst under reaction conditions that promote the formation of an aldehyde-rich crude reaction product; (b) demetalling the aldehyde-rich crude reaction product to recover therefrom the hydroformylation catalyst and a substantially catalyst-free, aldehyde-rich crude reaction product;

(c) separating the catalyst-free, aldehyde-rich crude reaction product into a concentrated aldehyde-rich product and an aldehyde-poor product;

(d) reacting the concentrated aldehyde-rich product either with (i) oxygen (optionally with a catalyst) or (ii) water in the presence of an acid-forming catalyst and in the absence of hydrogen, thereby converting the concentrated aldehyde- rich product into a crude acid product; and

(e) separating the crude acid product into a branched oxo acid and an acid-poor product.

The olefinic feedstream is preferably any C₄ to C₉ olefin, more preferably a branched C₇ olefin. Moreover, the olefinic feedstream is preferably a branched olefin, although a linear olefin which is capable of producing all branched oxo acids are also contemplated herein. The hydroformylation and subsequent reaction of the crude hydroformylation product with either (i) oxygen (e.g., air), or (ii) water in the presence of an acid-forming catalyst, is capable of producing branched Cs to Cio acids, more preferably branched Cg acid (i.e., Cekanoic^® 8 acid). Each of the branched oxo C₅ to Cio acids formed by the conversion of branched oxo aldehydes typically comprises, for example, a mixture of branched oxo acid isomers, e.g., Cekanoic^® 8 acid comprises a mixture of 26 wt% 3,5-dimethyl hexanoic acid, 19 wt% 4,5-dimethyl hexanoic acid, 17 wt% 3,4-dimethyl hexanoic acid, 1 1 wt% 5-methyl heptanoic acid, 5 wt% 4-methyl heptanoic acid, and 22 wt% of mixed methyl heptanoic acids and dimethyl hexanoic acids.

Any type of catalyst known to one of ordinary skill in the art which is capable of converting oxo aldehydes to oxo acids is contemplated by the present invention. Preferred acid-forming catalysts are disclosed in co-pending and commonly assigned U.S. Patent Application, Serial No. 08/269,420 (Vargas et al.), filed on June 30, 1994, and which is incorporated herein by reference. It is preferable if the acid- forming catalyst is a supported metallic or bimetallic catalyst. One such catalyst is a bimetallic nickel-molybdenum catalyst supported on alumina or silica alumina which catalyst has a phosphorous content of about 0.1 wt% to 1.0 wt%, based on the total weight of the catalyst. Another catalyst can be prepared by using phosphoric acid as the solvent for the molybdenum salts which are impregnated onto the alumina support. Still other bimetallic, phosphorous-free Ni/Mo catalyst may be used to convert oxo aldehydes to oxo acids.

Linear Acids

The preferred mono-and/or di-carboxylic linear acids are any linear saturated alkyl carboxylic acid having a carbon number in the range between about C2 to C20, preferably C2 to Cjo

Some examples of linear saturated acids include acitic, propionic, n- pentanoic, n-heptanoic, n-octanoic, n-nonanoic, and n-decanoic acids.

High Hydroxyl Esters

The "high hydroxyl" ester employed in the present invention has from about 1% to about 35% unconverted hydroxyl groups, based upon the total amount of hydroxyl groups in the alcohol.

A common technique for characterizing the conversion of hydroxyl groups is hydroxyl number. A standard method for measuring hydroxyl number is detailed by the American Oil Chemists Society as A.O.C.S., Cd 13-60. The ester of the present invention is characterized as having hydroxyl numbers ranging for about greater than 5 to about 180. The term "high hydroxyl," as used herein, refers to partially esterified esters characterized as having a hydroxyl number greater than about 5.

Fuels Additive

The high hydroxyl ester product of this invention can be used as a fuels additive by itself or in conjunction with other fuels additives such as detergents or dispersants, anti-oxidants, corrosion inhibitors, pourpoint depressants, color stabilizers, carrier fluids, solvents, and the like. The foregoing additives may provide a multiplicity of effects and are included herein to illustrate that the high hydroxyl ester of the present invention may be complimented by such additives. This approach is well known in the relevant art.

The high hydroxyl ester is suitable as an additive or admixture to a wide variety of motor fuels generally identified as hydrocarbons which boil in the gasoline boiling range of 80°-450°F. This includes straight or branched chain paraffins, cyclo- paraffins, olefins, oxygenates (including MTBE, ETBE, TAME and the like), aromatic hydrocarbons, alcohols (including methanol and ethanol), or mixtures thereof. The present invention is preferably suitable as a gasoline additive wherein gasoline is referred to in a general sense as comprising a mixture of liquid hydrocarbons or hydrocarbon oxygenates having an initial boiling point in the range from about 70 to 135°F and a final boiling point in the range from about 250 to 450°F as determined by the ASTM D86 distillation method.

The following examples describe specific formulations of high hydroxyl esters in distillate fuel, embodying the present invention.

Example 1

A polyol ester illustrative of the present invention was prepared in the following manner:

Cekanoic^®8 acid (4 moles, 576 g) and glycerol (2 moles, 184 g) were charged to an esterifi cation reactor under a slight positive nitrogen flow. The mixture was heated for about three hours to a maximum temperature of 220°C during which time about 69cc of H₂O were collected in a Dean-Stark Trap. At this time a vacuum was applied to strip any residual Cekanoic^®8 acid. After about two hours of stripping, the reaction mixture was cooled and a sample was taken and analyzed by gas chromatography. This analysis indicated the absence of glycerol and of Cekanoic^®8 acid. A total of 525 g of product were obtained. The sample exhibited a hydroxyl number of 79.

One of the important aspects of this invention is its reduction of IVD and CCD in internal combustion engines. Two types of bench tests as well as testing in internal combustion engines were utilized to determine and compare intake valve deposits and combustion chamber deposits. The STRIDE (Surrogate Test Related to Intake Deposits in Engines) and the TORID test are both bench tests which emulate the effects of fuels composition on intake valve deposits and combustion chamber deposits, respectively. The STRIDE test is described in detail in U.S. patent 5,492,005. The TORID test was developed by Exxon Research and Engineering Company and is used to emulate the effects of fuel additives on combustion chamber deposits. The TORID test differs from the STRIDE test inasmuch as the sample, which is controllably heated as in the STRIDE testing, is also subjected to a controlled flash flame to simulate combustion chamber conditions. The results of the STRIDE tests may be used to predict the effect of fuel additives on intake valve deposits; the TORID tests may be used to predict the effect of fuel additives on combustion chamber deposits. These bench tests data were validated, and correlations developed with results obtained through engine testing in conventional internal combustion engines.

In the STRIDE testing, the sample fuel was delivered at a rate of 10 ml/hour to a 0.3 cm^ stainless steel nub surface. The surface temperature was cycled from about 150°C to about 300°C. Cycle time was approximately 8 minutes. Total time of tests was approximately four hours.

In the TORID testing, approximately 2.0 mg of the sample additive are placed on a nub maintained at about 225°C and an amount of hexane is delivered and ignited at about 0.5 second intervals to simulate the combustion chamber flame. Total time of tests was approximately one hour.

Using the STRIDE and TORID testing, IVD and CCD emulation data were obtained for a number of samples having the high hydroxyl fuel additive and several commercially available ester fuel additives. Table 1 describes what types of samples were tested and the TORID and STRIDE test results. In the STRIDE test, compounds A through P were used at 500 wpp in base fuel. In the TORID test, the additives were evaluated in the absence of fuel. For the STRIDE testing, deposits less than base fuel predict reduced intake valve deposits (IVD). The TORID test is comparative and predicts the propensity of fuel additives to cause combustion chamber deposits. Table 1

STRIDE TORID

Deposits Deposits

Compound Description (microεrams) (microerams)

Base Fuel 500 N/A

A Ester of technical grade pentaerythritol alcohol with a 260 3 mixture of 3,5,5 trimethyl hexanoic acid and Cekanoic^®8 acid; Hydroxyl number = 19

B Ester of technical grade pentaerythritol alcohol with a 200 33 mixture of 3,5,5 trimethyl hexanoic acid and Cekanoic^®8 acid; Hydroxyl number = 125

Ester of technical grade pentaerythritol alcohol with a 250 40 mixture of 3,5,5 trimethyl hexanoic acid and Cekanoic*8; Hydroxyl number < 5

Ester of technical grade pentaerythritol alcohol with a 180 93 mixture of Cekanoic^®8 acid and linear Cg, C₎₀ acids. Hydroxyl number = 123

Ester of technical grade pentaerythritol alcohol with a 190 133 mixture of Cekanoic^®8 acid and linear C_g, do acids. Hydroxyl number < 5

Ester of technical grade pentaerythritol alcohol with 170 233 straight chain aliphatic carboxylic acids Hercolube F¹

Ester of trimethylolpropane with 3,5,5 trimethyl 373 hexanoic acid; Hydroxyl number = 110

H Ester of trimethylolpropane with mixture of linear Cg, 100 70 Cio acids; Hydroxyl number = 71

Ester of trimethylolpropane with mixture of linear Cg, 170 62 Cio acids; Hydroxyl number = 54

Ester of trimethylolpropane with mixture of linear Cg, 247 113 Cio acids; Hydroxyl number < 5 Priolube 970²

Ester of glycerol with Cekanoic^®8 acid; 190 Hydroxyl number = 79

Ester of glycerol with mixture of linear Cg, C₎₀ acids 420 45 Hydroxyl number = 72

M Ester of glycerol with mixture of linear C , C_!0 acids 400 115 Hydroxyl number = 5.8 Table 1 -cont.

STRIDE TORID

Deposits Deposits

Compound Description (microerams) (microerams) Base Fuel 500 N/A

N Ester of mixture of glycerol oleates; 810 530 Parabar 9440. Hydroxyl number 22 ³

O Ester of trimethylolpropane with Cekanoic^®8 acid, 420 0 Hydroxyl number < 5

Ester of trimethylolpropane with Cekanoic^®8 acid, 200 Hydroxyl number = 75

1. Hercolube F is a tradename of Hercules Inc.; this commercially available ester additive has a hydroxyl number of < 5.

2. Priolube 3970 is a trade name of Unichema; this commercially available ester additive has a hydroxyl number of < 5.

3. Parabar 9440 is a trade name of a mixture of glycerol oleatcs commercially available from Exxon Chemical Company; it has a hydroxyl number of 223.

Certain specified compounds identified in Table 1 were added to gasoline (base fuel) and subjected to actual internal combustion engine testing. Tables 2 and 3 show engine test results for IVD and CCD relative to the base fuel. These data evidence a reduction of IVD and CCD in the percentages shown relative to the base fuel.

Table 2 - Testing in Engine I

% Deposits Over (+) or Under (-) Base Fuel

Concentration in Fuel IVD CCD

Gasoline Additive (wppm) % Compared % Compared with with Base Fuel Base Fuel

Compound A in base fuel 1,500 (-) 70 0

Compound N in base fuel 1.500 (-) 21 (+) 77 Table 3 - Testing in Engine 2

% Deposits Over (+) or Under (-) Base Fuel

Concentration in Fuel IVD CCD

Gasoline Additive (wppm) % Compared % Compared with Base Fuel with Base Fuel

Compound G in base fuel 500 M 19 Oil

Another feature of the present invention is improved wear and friction performance, both in the fuel delivery system (such as fuel pump and fuel injection system) and within the combustion chamber and piston/cylinder assembly by surficial coating of the operating surfaces. Yet another aspect of the present invention is that the high hydroxyl ester additive does not deteriorate the crankcase lubricant when amounts of the fuel additive reach the engine's crankcase lubricant. To the contrary, the high hydroxyl ester of the present invention is understood to survive the combustion chamber conditions and, upon passing into the crankcase improves the wear and frictional performance of the crankcase oil.

To demonstrate the foregoing aspects of this invention, the "used" engine oil of the Engine 1 Testing described in Table 2 was tested using a Falex Block-on-Ring tribometer. This test, well known in the art, permits measurement of friction coefficients between two metal surfaces under differing conditions of temperature, speed and load to map a wide range of boundary and mixed lubrication conditions. The loads vary from low (LL) of 1 10 lbs. to high (HL) of 220 lbs., and low speed (LS) of 105 rpm to high speed (HS) of 420 rpm. The friction coefficient data are shown in Figure 1 in comparison to data for oil from the same engine, operated under substantially similar conditions, except without the high hydroxyl ester fuel additive of the present invention (Base Run). The Falex Block-on-Ring tribometer was also equipped with a proprietary eddy current sensor to measure the wear rate of the block during the test. Figure 2 exhibits wear rates of the "used" engine oils and shows improved performance in the case of the oil from the engine run with the fuel containing the high hydroxyl ester.

Claims

CLAIMS:

1. A fuel composition for use in internal combustion engines comprising a major amount of gasoline and a minor amount of an ester comprising the reaction product of:

an alcohol having the general formula R(OH)n, where R is an aliphatic group, cyclo-aliphatic group or combination thereof having from about 2 to 20 carbon atoms and n is at least 2 and the alipatic group is a branched or linear aliphatic group; and

at least one branched and/or linear saturated acid which has a carbon number in the range between about C2 to C20; wherein said synthetic ester composition is characterized as having a hydroxyl number greater than about 5 and less than about 180.

2. The fuel composition according to claim 1 wherein said acid is a branched monocarboxylic acid.

3. The fuel composition according to claim 2 wherein said branched monocarboxylic acid is any mono-carboxylic acid which has a carbon number in the range of about C4 to C20

4. The fuel composition according to claim 3 wherein said branched mono-carboxylic acid has a carbon number in the range of about C5 to Cio.

5. The fuel composition according to claim 2 wherein said acid is selected from the group consisting of 2,2-dimethylpropionic acid, neoheptanoic acid, neooctanoic acid, neononanoic acid, isohexanoic acid, neodecanoic acid, 2-ethyl hexanoic acid, 3,5,5-trimethyl hexanoic acid, isoheptanoic acid, isooctanoic acid, isononanoic acid, 2-methylbutyric acid and isodecanoic acid, or mixtures thereof.

6. The fuel composition according to claim 2 wherein said branched mono-carboxylic acid is an isooctanoic acid.

7. The fuel composition according to claim 1 wherein said linear acid is any linear saturated alkyl carboxylic acid having a carbon number in the range between about C2 to C20-

8. The fuel composition according to claim 7 wherein said linear acid is any linear saturated alkyl carboxylic acid having a carbon number in the range between about C2 to C \Q.

9. The fuel composition of claim 8 wherein said linear acid is selected from the group consisting of acetic, propionic, n-pentanoic, n-heptanoic, n-octanoic, n-_* nonanoic, and n-decanoic acids, or mixtures thereof.

10. The fuel composition according to claim 1 wherein said alcohol is selected from the group consisting of: neopentyl glycol, 2,2-dimethylol butane, trimethylol ethane, trimethylol propane, trimethylol butane, mono-pentaerythritol, technical grade pentaerythritol, di-pentaerythritol, tri-pentaerythritol, ethylene glycol, propylene glycol, polyalkylene glycols, 1,4-butanediol, sorbitol, and 2- methylpropanediol, or mixtures thereof.

1 1. The fuel composition of claim 1 wherein said ester composition comprises from about 10 wppm to about 10,000 wppm of said fuel composition.