CN1230210A

CN1230210A - Polyol ester fuels additive

Info

Publication number: CN1230210A
Application number: CN97197899A
Authority: CN
Inventors: E·P·维拉霍普劳; J·E·约翰斯顿; R·H·施罗斯伯格; S·R·凯勒曼; M·西斯肯
Original assignee: ExxonMobil Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1996-09-13
Filing date: 1997-09-11
Publication date: 1999-09-29
Also published as: CA2264707A1; EP0948586A4; EP0948586A1; AU4416697A; AU734585B2; WO1998011177A1; BR9711783A; JP2001501231A

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 fuel additive

Technical Field

The present invention relates to a polyol ester additive for fuels, and more particularly, to a fuel additive comprising a partially esterified polyol ester which reduces engine fouling, including intake valve fouling ("IVD") and combustion chamber fouling ("CCD"), and improves wear and friction properties of materials in contact therewith, including crankcase lubricating oils. The polyol ester fuel additives of the present invention have unconverted hydroxyl groups derived from the reaction product of a polyol and a branched acid, a straight chain saturated acid, or mixtures thereof.

Background

Fuel formulations for internal combustion engines are becoming increasingly complex. Engine fuels essentially require the addition of additives to reduce carbonaceous deposits on valves and in combustion chambers, protect fuel injector orifices and other fuel delivery components such as fuel pumps, provide corrosion resistance and residual liquid resistance, and accomplish a variety of other objectives.

Esters generally have excellent thermal and oxidative stability and are therefore widely used in synthetic or partially synthetic crankcase lubricating oils. Recent developments in the prior art have shown that esters have the potential to be used as fuel additives. For example, US patent 5,427,591 discloses that the use of certain hydroxyaromatic esters as fuel additives can reduce engine carbon deposition. US patent 5,211,721 discloses the use of polyalkylene esters as fuel additives to control engine fouling and improve fuel fogging. U.S. Pat. No. 5,089,028 teaches that engine intake valves, valve fuel injectors and carburetors can be cleaned and acted as corrosion inhibitors using a mixture of polyalkenyl succinimides, olefin polymers, esters and polyethers.

However, these fuel additives generally lose their effectiveness and, while achieving a certain end, negatively impact other aspects of engine performance characteristics. Thus, a variety of additives must be employed to achieve the desired overall performance enhancement. For example, US patent 5,433,755 discloses a multi-component additive that achieves fuel cleanliness and corrosion resistance. The prior art also shows that high molecular weight esters allow combustion to take place in the cylinder, thereby providing a surface lubricant effect to the cylinder wall and piston rings, while low molecular weight esters provide a cleaning effect such as injector soot reduction. US patent 4,920,691 teaches that the combined use of a low molecular weight linear carboxylic acid ester (i.e., molecular weight less than 200) and a high molecular weight linear carboxylic acid ester (i.e., molecular weight 300-1000) achieves both cleaning and cylinder wall lubrication. In addition to the increased cost of fuel, it has also been found that the amount of detergent additives used is minimized because the by-products of such additives have a detrimental effect on the crankcase lubricant; for example, referring to US patent 5,044,478, small amounts of by-products of such additives, after breakdown in the combustion chamber, will roll up on the crankcase lubricating oil and cause damage to the engine oil. The fuel additives of the present invention have been shown to improve IVD and CCD while not presenting the lubricating oil problems associated with known additives. In contrast, the formulations of the present invention improve the wear and friction properties of crankcase lubricating oils.

Summary of The Invention

The present inventors have developed a unique fuel additive that utilizes polyol esters of polyols synthesized with branched acids, saturated linear acids or mixtures thereof in such a way that the resulting esters have unconverted hydroxyl groups. The resulting fuel compositions exhibit enhanced intake valve soot ("IVD") and combustion chamber soot ("CCD") control, and reduced wear and friction properties of the fuel lines, combustion chambers, and piston/cylinder assemblies. While improving IVD and CCD, crankcase lubricating oils also do not suffer from the drawbacks associated with commonly used detergent additives. In contrast, the fuel additives of the present invention may be present in the combustion chamber, improving wear and friction properties of the crankcase lubricating oil. The esters comprise compounds having the general formula R (OH)_nWherein R is an aliphatic group, cycloaliphatic group, or combinations thereof, R has from about 2 to about 20 carbon atoms, n is at least 2, and said aliphatic group is a branched or straight chain group. The esters are characterized by a hydroxyl number greater than about 5 to about 180. The fuels referred to in this invention typically comprise distillate fuels and primarily comprise gasoline. The fuel contains a small amount of ester additive, about 10 to 10,000 wppm.

Drawings

Figure 1 shows the coefficient of friction under various load, speed and temperature conditions.

Figure 2 shows wear rate data under various load, speed and temperature conditions.

Detailed Description

The fuel compositions of the present invention employ a polyol ester comprising a polyol of the formula R (OOCR')_nA compound of formula (I) and at least one of the following compounds:

R(OOCR′)_n-1OH

R(OOCR′)_n-2(OH)₂and are and

R(OOCR′)_n-(i)(OH) (i) wherein n is an integer of at least 2, R is an aliphatic or cycloaliphatic radical containing from about 2 to about 20 or more carbon atoms, or a combination thereof, wherein said aliphatic radical is branched or straight chain; r' is a branched or straight chain hydrocarbon group having about 2 to 20 carbon atoms and (i) is an integer from 0 to n. Unless otherwise specified, the polyol ester composition may also include an excess of R (OH)_n。

The esters are preferably formed by reacting a polyol (i.e., a polyol) with at least one branched or straight chain saturated acid or mixtures thereof. By adjusting the composition of the feed polyol to obtain the desired product ester composition.

The esterification reaction is preferably carried out under the following reaction conditions: with or without the use of a catalyst, at a temperature of about 140 ℃ and 250 ℃, at a pressure in the range of 30-760mmHg, and for a reaction time of about 0.1-12 hours, preferably 1-8 hours. In a preferred embodiment, the reactor unit may be vacuum stripped to remove acid in order to optimize the final composition. The product may then be treated in a contact treatment step, i.e. by mixing the product with a solid such as alumina, zeolitic activated carbon or clay, etc. Alcohol(s)

The alcohols used for the reaction with the branched and/or saturated linear acids are of the general formula R (OH)_nThe polyhydroxy compounds represented by (A) are,wherein R is an aliphatic or cycloaliphatic radical or a combination thereof, an aliphatic radical or a branched or straight chain radical, and n is at least 2. The hydrocarbyl group may contain about 2 to 20 or more than 20 carbon atoms and is preferably an alkyl group. The hydroxyl groups may be separated by one or more carbon atoms.

The polyol may typically contain one or more oxyethylene groups and thus the polyol includes compounds such as polyether polyols.

The following alcohols are particularly suitable as polyols in the practice of the present invention: neopentyl glycol, 2-dimethylolbutane, trimethylolethane, trimethylolbutane, monopentaerythritol, technical grade pentaerythritol, dipentaerythritol, tripentaerythritol, ethylene glycol, propylene glycol, and polyalkylene glycols (e.g., polyethylene glycol, polypropylene glycol, 1, 4-butanediol, sorbitol, and the like, 2-methylpropanediol, polytetramethylene glycol, and the like, and blends such as oligomeric mixtures of ethylene glycol and propylene glycol). The most preferred alcohols are technical grade pentaerythritol (e.g., about 88% mono, 10% di, and 1-2% tripentaerythritol), monopentaerythritol, dipentaerythritol, neopentyl glycol, and trimethylolpropane. Branched acids

The branched acid is preferably a monocarboxylic acid having about 4 to 20 carbon atoms, more preferably about 5 to 10 carbon atoms, with methyl or ethyl branching being preferred the monocarboxylic acid is preferably at least one selected from the group consisting of 2, 2-dimethylpropionic acid (pivalic acid), neoheptanoic acid, neooctanoic acid, neononanoic acid, isocaproic acid, neodecanoic acid, 2-ethylhexanoic acid (2EH), 3,5, 5-trimethylhexanoic acid (TMH), isoheptanoic acid, isooctanoic acid, isononanoic acid, and isodecanoic acid the particularly preferred branched acid is 3,5, 5-trimethylhexanoic acid the term "neo" herein refers to a trialkylacetic acid, i.e., an acid substituted three times at the α carbon atom position with alkyl groups equal to or greater than CH₃The structure is shown in the following general formula:

wherein R is₁、R₂And R₃Greater than or equal to CH₃Not equal to hydrogen.

3,5, 5-trimethylhexanoic acid has the following structural formula:branched chain oxoacids

Branched oxo acids are preferably mono oxo carboxylic acids having about 5 to 10 carbon atoms, preferably 7 to 10 carbon atoms, of which methyl branches are preferred. The mono-oxygen-containing carboxylic acid is at least one selected from the following acids: isovaleric acid, isocaproic acid, isoheptanoic acid, isooctanoic acid, isononanoic acid, and isodecanoic acid. In particularA preferred branched chain oxoacid is isooctanoic acid, known under the tradename Cekanoic^®8 acids, commercially available from Exxon chemical company. Another particularly preferred branched oxoacid is 3,5, 5-trimethylhexanoic acid, a form of which is also commercially available from Exxon chemical under the tradename Cekanoic^®9 or Ck^®9。

The term "iso" refers to the product of the multiple isomers that are transported by carbonylation. Preferably, the branched oxoacids have multiple isomers, preferably more than three isomers, and most preferably more than 5 isomers.

Branched oxo acids can be produced in a so-called "carbonylation" process, i.e. by reacting commercial C₄-C₉Hydroformylation of an olefin fraction to give the corresponding C-containing branch₅-C₁₀-the carbonylation reaction product of an aldehyde. In the reaction process for forming the oxo acid, it is preferred to form an aldehyde intermediate from the carbonylation reaction product and then convert the crude oxo aldehyde product to the oxo acid.

In order to produce oxo acids industrially, the hydroformylation process has to be adjusted to maximize the production of oxo aldehydes. This can be achieved by controlling the temperature, pressure, catalyst concentration and/or reaction time. The demetallized crude aldehyde product is then distilled to remove the oxo-alcohols from the oxo-aldehydes, and the oxo-aldehydes are then oxidized to produce the desired oxo-acids according to the following reaction:

(1) wherein R is a branched alkyl group.

Alternatively, the oxo acids may also be formed by: the demetallized crude aldehyde product is reacted with water in the presence of an acid forming catalyst in the absence of hydrogen at a temperature of from about 93 ℃ to about 205 ℃ and a pressure of from about 0.1 Mpa to about 6.99Mpa, and the concentrated aldehyde-rich product is converted to a crude acid product, which is then separated into an acid-rich product and an acid-poor product.

A process for the production of branched chain oxo acids from an olefin feed by cobalt catalysed hydroformylation comprises the steps of:

(a) reacting an olefin feed with carbon monoxide and hydrogen (i.e., syngas) in the presence of a hydroformylation catalyst under conditions that promote formation of an aldehyde-rich crude reaction product to hydroformylate the olefin feed;

(b) demetallizing the crude aldehyde-rich product to recover the hydroformylation catalyst and a substantially catalyst-free crude aldehyde-rich reaction product;

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

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

(e) the crude acid product is separated into branched chain oxo acids and an acid depleted product.

The olefin feed is preferably any C₄-C₉-olefins, more preferably branched C7 olefins. Further, the olefin feed is preferably a branched olefin, although the present invention contemplates the use of linear olefins that produce all branched oxo acids. Hydroformylation and subsequent reaction of the crude hydroformylation product with (i) oxygen (e.g. air) or (ii) water in the presence of an acid-forming catalyst to form a branched C₅-C₁₀-acids, more preferably branched C₈Acids (i.e. Cekanoic)^®8 acids). Branched oxygen-containing C formed by conversion of branched oxygen-containing aldehydes₅-C₁₀Acids usually comprise mixtures of branched oxoacid isomers, e.g. Cekanoic^®The 8-acid comprises 26 wt% of 3, 5-dimethylhexanoic acid, 19 wt% of 4, 5-dimethylhexanoic acid, 17 wt% of 3, 4-dimethylhexanoic acid, 11 wt% of 5-methylheptanoic acid, 5 wt% of 4-methylheptanoic acid and 22 wt% of a mixture of mixed methylheptanoic and dimethylhexanoic acids.

Any catalyst known to those skilled in the art that is capable of converting an oxygen-containing aldehyde to an oxo acid may be used in the present invention. Preferred acid forming catalysts are disclosed in co-pending U.S. patent application 08/269,420(vargas et al), filed 6/30, 1994, which is incorporated herein by reference. The acid-forming catalyst preferably employed is a supported metal or bimetallic catalyst. One of the catalysts is a bimetallic nickel-molybdenum catalyst on alumina or silica-alumina having a phosphorous content of about 0.1 to 1.0 wt.%, based on the total weight of the catalyst. Another catalyst can be prepared by: phosphoric acid is used as a solvent for the molybdenum salt, which is impregnated on the alumina support. Other bimetallic phosphorus-free nickel/molybdenum catalysts may also be used to convert oxygen-containing aldehydes to oxo acids. Straight chain acid

Preferred mono-and/or di-basic straight chain carboxylic acids are any straight chain saturated alkyl carboxylic acid having about 2 to 20 carbon atoms, preferably 2 to 10 carbon atoms.

Some examples of straight chain saturated acids include acetic acid, propionic acid, n-pentanoic acid, n-heptanoic acid, n-octanoic acid, n-nonanoic acid, and n-decanoic acid. High hydroxyl ester

"high hydroxyl" esters useful in the present invention have about 1-35% unconverted hydroxyl groups, based on the total amount of hydroxyl groups in the alcohol. A conventional technique for characterizing hydroxyl conversion characteristics is the hydroxyl number. Standard methods for measuring hydroxyl number are described in detail by the american oil chemistry association as a.o.c.s., Cd 13-60. The esters of the present invention are characterized as having a hydroxyl number of from about greater than 5 to about 180. The term "high hydroxyl" as used herein refers to partially esterified esters characterized by a hydroxyl number greater than about 5. Fuel additive

The high hydroxyl esters of the present invention are useful as fuel additives, either alone or in combination with other fuel additives such as detergents or dispersants, antioxidants, corrosion inhibitors, pour point depressants, color stabilizers, transfer fluids, solvents, and the like. The inclusion of these additives in the present invention illustrates that the high hydroxyl esters of the present invention can be supplemented by these additives to provide multiple benefits. Such a method is well known in the related art.

The high hydroxyl esters are useful as additives or blends for various motor fuels as are hydrocarbons boiling in the gasoline boiling range of 80-450F. The hydrocarbons include straight or branched chain paraffins, naphthenes, olefins, oxygenated hydrocarbons (oxygenates) (including METB, ETBE, TAME, etc.), aromatics, alcohols (including methanol and ethanol), or mixtures thereof. The present invention is also preferably suitable as a gasoline additive, wherein gasoline is generally defined as a mixture comprising a liquid hydrocarbon or oxygenated hydrocarbon having an initial boiling point of about 70-135 ° F and a final boiling point of about 250-450 ° F, as measured by the astm d86 distillation method.

The following examples illustrate specific formulations of high hydroxyl esters useful in the present invention in distillate fuels. Example 1

The polyol esters of the present invention are prepared in the following manner:

under a slight nitrogen purge, Cekanoic^®8 acids (4mol,576g) were added to the esterification reactor together with glycerol. The mixture was heated for about 3 hours to a maximum temperature of 220 ℃, during which about 69ml of water was collected in the dean-stark trap. At this point, vacuum was applied to the trap to strip out residual Cekanoic^®8, and (3) acid. After stripping for about 2 hours, the reaction mixture was cooled,samples were taken and analyzed by gas chromatography. Analysis showed the absence of glycerol and Cekanoic^®8, and (3) acid. The total amount of product obtained was 525 g. The sample showed a hydroxyl number of 79.

One weight aspect of the present invention is that IVD and CCD are reduced in an internal combustion engine. Two types of bench tests and tests in an internal combustion engine were performed to determine and compare intake valve fouling and combustion chamber fouling. Both the string (Surrogate test associated with intake valve fouling in engines) and the torrid test are small tests that simulate the effect of fuel composition on intake valve fouling and combustion chamber fouling, respectively. The STRIDE test is described in detail in US patent 5,492,005. The TORID test was developed by Exxon Research and Engineering and was used to simulate the effect of fuel additives on combustion chamber carbon deposition. The TORID test differs from the STRIDE test in that the controlled heated specimens also undergo controlled rapid combustion in the STRIDE test to simulate combustion chamber conditions. The results of the STRIDE test can be used to predict the effect of the fuel additive on carbon deposition in the intake valve; and the TORID test can be used to predict the effect of fuel additives on combustion chamber soot. These small test data were confirmed and the correlation was concluded with the results obtained by engine tests in a conventional internal combustion engine.

In the STRIDE test, a fuel sample is delivered at a rate of 10ml/h to 0.3cm²On the surface of a small piece of stainless steel. The surface temperature is cycled between about 150 ℃ and 300 ℃. The cycle time was approximately 8 minutes. The total test time was approximately 4 hours.

In the TORID test, approximately 2.0mg of an additive sample was placed on a small block maintained at about 225℃, and an amount of hexane was delivered to ignite at intervals of about 0.5 seconds, simulating a combustion chamber flame. The total test time was approximately 1 hour.

IVD and CCD simulation data were obtained for a large number of samples with high hydroxyl fuel additives and several samples of commercially available ester fuel additives using the STRIDE and TORID tests. Table 1 shows the types of samples tested and the results of the TORID and STRIDE tests. In the STRIDE test, compounds A to P were used at 500wppm in the base combustion. In the TORID test, the additives are evaluated in the absence of fuel. For the STRIDE test, less soot is present than the base fuel predicted intake valve soot (IVD) reduction. While the TORID test is comparative, it is predicted that fuel additives tend to cause combustion chamber carbon build-up.

TABLE 1

STRIDE TORID COMPOUNDS demonstrate soot (microgram) base fuel 500N/A A technical grade pentaerythritol and 3,5, 5-trimethylhexanoic acid 2603

And Cekanoic^®8 esters of acid mixtures; hydroxyl radical

Radix = 19B technical grade pentaerythritol and 3,5, 5-trimethylhexanoic acid 20033

And Cekanoic^®8 esters of acid mixtures; hydroxyl radical

Radix = 125C technical grade pentaerythritol and 3,5, 5-trimethylhexanoic acid 25040

And Cekanoic^®8 esters of acid mixtures; hydroxyl radical

Basic number < 5D technical grade pentaerythritol and Cekanoic^®8 acid 18093

And straight chain C₈,C₁₀Esters of acid mixtures; number of hydroxyl groups

= 123E technical grade pentaerythritol and Cekanoic^®8 acid 190133

Industrial grade pentaerythritol & lt 5F and linear aliphatic carboxylic acid 170233

Esters of Hercolube F¹3730 of G-trimethylolpropane and 3,5, 5-trimethylhexanoic acid

An ester; hydroxyl number = 110H trimethylolpropane and linear C₈,C₁₀Acid mixture 10070

Esters of (a); hydroxyl number = 71I trimethylolpropane and linear C₈,C₁₀Acid mixture 17062

Esters of (a); hydroxyl number = 54J trimethylolpropane and linear C₈,C₁₀Acid mixture 247113

Esters of (a); hydroxyl number < 5

Priolube 3970²

K Glycerol and Cekanoic^®8 acid esters; hydroxy 1900

Number =79

L Glycerol and Linear C₈,C₁₀Esters of acid mixtures; hydroxy 42045

Cardinality =72

M Glycerol and Linear C₈,C₁₀Esters of acid mixtures; hydroxyl radical 400115

Cardinality =5.8

Esters of mixtures of N glyceryl oleates; 810530

Parabar9440, hydroxyl number 223³

O-trimethylolpropane and Cekanoic^®4200 acids of 8

Esters, number of hydroxyl groups < 5

P-trimethylolpropane and Cekanoic^®2000 of 8 acids

Ester, hydroxyl number = 751. Hercolube F is a trade name of the product from Hercules inc; the hydroxyl number of such commercially available ester additives is less than 5. Priolube3970 is the trade name of Unichema products; the hydroxyl number of this commercially available ester additive is less than 5. Parabar9440 is a trade name for a glycerol oleate mixture commercially available from Exxon chemical company; the hydroxyl number is 223.

Some of the specific compounds identified in table 1 were added to gasoline (base fuel) and subjected to actual internal combustion engine tests. Tables 2 and 3 show the engine test results for IVD and CDD relative to the base fuel. These data demonstrate the percent reduction in IVD and CDD relative to the base fuels.

TABLE 2 Engine 1 test

	Carbon deposition Rate% (+)	Higher than base fuel, ()	Lower than base fuel
	Carbon deposition Rate% (+)	Higher than base fuel, ()	Lower than base fuel	Gasoline additive	Concentration in fuel (wppm)	Compared with the base fuel IVD％	Compared with the base fuel CCD％
Compound A is at the base In fuel	1,500	(-)70	0	Gasoline additive	Concentration in fuel (wppm)	Compared with the base fuel IVD％	Compared with the base fuel CCD％
Compound A is at the base In fuel	1,500	(-)70	0	Compound N is on the basis In fuel	1,500	(-)21	(+)77

TABLE 3 Engine 2 test

	Has a carbon deposition rate higher than that of the basic fuel and lower than that of the basic fuel
				Gasoline additive	Concentration in fuel (wppm)	Compared with the base fuel IVD％	Compared with the base fuel CCD％
Compound G is on the basis In fuel	500	(-)19	(-)13	Gasoline additive	Concentration in fuel (wppm)	Compared with the base fuel IVD％	Compared with the base fuel CCD％

Another feature of the present invention is that wear and friction performance in fuel delivery systems (e.g., fuel pumps and fuel injection systems) and in combustion chambers and piston/cylinder assemblies may be improved by surface coating of the operative surfaces. It is another aspect of the present invention that the high hydroxy ester additive does not damage the crankcase lubricating oil when the amount of fuel additive reaches the amount of crankcase lubricating oil for the engine. In contrast, the high hydroxyl esters of the present invention are understood to be present in the combustion chamber conditions and pass through the crankcase, improving the wear and friction properties of the crankcase lubricating oil.

To illustrate the above aspects of the invention, the "used" engine oil of Engine 1 test set forth in Table 2 was tested using a Falex Block-on-Ring tribometer. This test is well known and measures the coefficient of friction between two metal surfaces at different temperature, speed and load conditions reflecting boundary and mixed lubrication conditions. The load value was changed from a low value of 110 pounds (LL) to a high value of 220 pounds (HL), with the speed changed from a low speed of 105rpm (LS) to a high speed of 420rpm (HS). FIG. 1 shows coefficient of friction data comparing data obtained under substantially similar conditions for oils from the same engine, except that the high hydroxyl ester fuel additive of the present invention was not used (base oil test). The Falex Block-on-Ring tribometer is also equipped with a proprietary eddy current sensor to measure the wear rate of the slider during the test. FIG. 2 shows the wear rate of "used" engine oil and shows that oil from an engine run on a fuel containing a high hydroxyl ester has improved performance.

Claims

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

a general formula R (OH)_nWherein R is an aliphatic or cycloaliphatic radical containing from about 2 to about 20 carbon atoms, or a combination thereof, and n is an integer of at least 2, wherein said aliphatic radical is a branched or straight chain aliphatic radical; and

at least one branched and/or straight chain saturated acid having from about 2 to about 20 carbon atoms; wherein said synthetic ester composition is characterized by a hydroxyl number greater than about 5 and less than about 180.

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

3. The fuel composition of claim 2 wherein said branched monocarboxylic acid is any monocarboxylic acid having from about 4 to about 20 carbon atoms.

4. The fuel composition of claim 3 wherein said branched monocarboxylic acid has from about 5 to about 10 carbon atoms.

5. The fuel composition of claim 2 wherein said acid is selected from the group consisting of: 2, 2-dimethylpropionic acid, neoheptanoic acid, neooctanoic acid, neononanoic acid, isocaproic acid, neodecanoic acid, 2-ethylhexanoic acid, 3,5, 5-trimethylhexanoic acid, isoheptanoic acid, isooctanoic acid, isononanoic acid, 2-methylbutyric acid and isodecanoic acid or mixtures thereof.

6. The fuel composition of claim 2 wherein said branched monocarboxylic acid is isooctanoic acid.

7. The fuel composition of claim 1 wherein said linear acid is any linear saturated alkyl carboxylic acid having from about 2 to about 20 carbon atoms.

8. The fuel composition of claim 7 wherein said linear acid is any linear saturated alkyl carboxylic acid having from about 2 to about 10 carbon atoms.

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

10. The fuel composition of claim 1, wherein said alcohol is selected from the group consisting of: neopentyl glycol, 2-dimethylolbutane, trimethylolethane, trimethylolpropane, trimethylolbutane, monopentaerythritol, technical pentaerythritol, dipentaerythritol, tripentaerythritol, ethylene glycol, propylene glycol and polyalkylene glycols, 1, 4-butanediol, sorbitol and 2-methylpropanediol, or mixtures thereof.

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