Patent Publication Number: US-2007094919-A1

Title: Fuel compositions

Description:
FIELD OF THE INVENTION  
      The present invention relates to fuel compositions comprising a gas oil base fuel.  
     BACKGROUND OF THE INVENTION  
      Two different fuel components can be blended so as to modify the properties and/or the performance, e.g. engine performance, of the resultant composition.  
      Diesel fuel components can include the so-called “biofuels” which derive from biological materials. Examples include levulinate esters.  
      Levulinate esters (esters of levulinic acid) and their preparation by reaction of the appropriate alcohol with furfuryl acetate are described in Zh. Prikl. Khim. (Leningrad) (1969) 42(4), 958-9, and in particular the methyl, ethyl, propyl, butyl, pentyl and hexyl esters.  
      WO-A-94/21753 discloses fuels for internal combustion engines, including both gasoline and diesel fuel, containing proportions (e.g. 1 to 90% v, 1 to 50% v, preferably 1 to 20% v) of esters of C 4-6  keto-carbonic acids, preferably levulinic acid, with C 1-22  alcohols. Esters with C 1-8  alcohols are described as being particularly suitable for inclusion in gasolines, and esters with C 9-22  alcohols are described as being particularly suitable for inclusion in diesel fuels. The examples in WO-A-94/21753 are about the inclusion of quantities of levulinate esters in gasolines, for improvement in octane numbers (RON and MON).  
      WO-A-03/002696 discloses a fuel composition incorporating levulinic acid, or a functional derivative thereof, with the object of providing more oxygen by volume than ethanol or traditional oxygenates such as MTBE or ETBE, giving little or no increase in fuel Reid vapour pressure and little or no effect on the flash point of the base fuel. The functional derivative is preferably an alkyl derivative, more preferably a C 1-10  alkyl derivative. Ethyl levulinate is said to be preferred, with methyl levulinate a preferred alternative. The levulinic acid or functional derivative is preferably used to form 0.1 to 5% v of the fuel.  
      Current commercially available compression ignition (diesel) engines tend to be optimised to run on fuels having a desired specification. Moreover, the conditions under which the engine is required to operate can affect the manner in which a fuel composition in the engine will behave. In particular, as the atmospheric temperature falls, a fuel that is a single-phase homogeneous liquid at normal temperatures may become a multiphase liquid as certain components either (i) freeze (forming solid wax) or (ii) become immiscible in the bulk liquid and form a separate liquid layer. The onset of wax formation on cooling is characterised by a change in the transparency of the fuel and the temperature at which this occurs is termed the “Cloud Point” of the fuel. If, on cooling, the Cloud Point is preceded by the formation of a separate liquid phase, the temperature at which this occurs is termed the “Phase separation temperature”. Diesel fuel specifications such as ASTM D975-02 (USA) and EN590 (Europe) include limits on Cloud Point temperature in order to ensure that diesel fuel remains fluid at the lowest anticipated service temperature and that blocking of fuel filters by wax is prevented. For trouble free operation, it is also desirable that the diesel fuel in the fuel tank remains homogeneous, since the composition of some or all of any separated liquid layers may be unsuitable as a fuel for the engine. The blending of a standard commercial diesel base fuel with other fuel components, to modify the overall fuel properties and/or performance, can therefore have an adverse impact on the performance of the blend in the engines for which it is intended.  
      For the above reason, it is desirable for any diesel fuel blend to have an overall specification as close as possible to that of the standard commercially available diesel base fuels for which engines tend to be optimised.  
      This can, however, be difficult to achieve because any additional fuel component is likely to alter the properties and performance of the base fuel. Moreover the properties of a blend, in particular its effect on low temperature performance, are not always straightforward to predict from the properties of the constituent fuels alone.  
     SUMMARY OF THE INVENTION  
      In one embodiment of the invention, there is provided a fuel composition comprising a gas oil base fuel, an alkyl levulinate and a co-solvent having a polar interaction parameter (δp) in the range of from 1 to 7 MPa 1/2  and a hydrogen bonding parameter (δh) in the range of from 2 to 18 MPa 1/2 .  
      In another embodiment of the invention, there is provided a method of reducing the phase separation temperature of a fuel composition comprising a gas oil base fuel and an alkyl levulinate comprising incorporating in the fuel composition a co-solvent having a polar interaction parameter (δp) in the range of from 1 to 7 MPa 1/2  and a hydrogen bonding parameter (δh) in the range of from 2 to 18 MPa 1/2 .  
      In yet another embodiment of the invention, there is provided a method of reducing the phase separation temperature of a fuel composition comprising a gas oil base fuel and an alkyl levulinate comprising (a) selecting by reference to its polar interaction parameter (δp) and hydrogen bonding parameter (δh) a co-solvent for which said polar interaction parameter is in the range of from 1 to 7 MPa 1/2  and said hydrogen bonding parameter is in the range of from 2 to 18 MPa 1/2  and (b) incorporating said selected co-solvent in the fuel composition.  
      Further there is provided a method to operate an engine and/or a vehicle comprising such fuel. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The relative solvency behaviour of a solvent can be expressed by a solubility parameter. One such parameter is the Hildebrand Solubility Parameter (HSP), which is defined as the square root of the molar cohesive energy, C, i.e. 
 
δHSP=C 1/2 ={( H   vap   −RT )/ V   m } 1/2 , 
 
 where H vap =molar heat of vaporisation, R=universal gas constant, T=temperature and V m =molar volume. 
 
      The Hildebrand parameter is an overall parameter, but Hansen was able to split it into three different molecular interactions, a dispersive interaction δd (non-permanent dipole-dipole interaction), a polar interaction δp (permanent dipole) and a hydrogen bonding interaction δh, their units being (cal/ml) 1/2  or MPa 1/2 , i.e. 
 
δHSP 2 =(δ d ) 2 +(δ p ) 2 +(δ h ) 2  (cal/ml or MPa) 
 
      It has now been found that in fuel compositions comprising a gas oil base fuel and an alkyl levulinate, the phase separation temperature of the fuel composition can be reduced by the inclusion of a co-solvent, preferably an alcohol, having a polar interaction parameter (δp) and a hydrogen bonding parameter (δh) falling within certain respective ranges. Such parameters can be found in, for example, “ Hansen Solubility Parameters: A user&#39;s handbook ”, C. M. Hansen, 2000, CRC Press, ISBN 0-8493-1525-5. In cases where the solubility parameters are not tabulated in that reference, it provides a suitable method for estimation.  
      In accordance with one embodiment of the present invention there is provided a fuel composition comprising a gas oil base fuel, an alkyl levulinate and a co-solvent, preferably an alcohol, which co-solvent has a polar interaction parameter (δp) in the range of from 1 to 7 MPa 1/2 , preferably from 1.5 to 6.5 MPa 1/2 , more preferably from 2.5 to 6 MPa 1/2 , and a hydrogen bonding parameter (δh) in the range of from 2 to 18 MPa 1/2 , preferably from 4 to 17 MPa 1/2 , more preferably from 6 to 16 MPa 1/2 .  
      In accordance with another embodiment of the present invention there is also provided a method of reducing the phase separation temperature of a fuel composition comprising a gas oil base fuel and an alkyl levulinate, which method comprises incorporating in the fuel composition a co-solvent, preferably an alcohol, which co-solvent has a polar interaction parameter (δp) in the range of from 1 to 7 MPa 1/2 , preferably from 1.5 to 6.5 MPa 1/2 , more preferably from 2.5 to 6 MPa 1/2 , and a hydrogen bonding parameter (δh) in the range of from 2 to 18 MPa 1/2 , preferably from 4 to 17 MPa 1/2 , more preferably from 6 to 16 MPa 1/2 .  
      In accordance with yet another embodiment of the present invention there is further provided a method of reducing the phase separation temperature of a fuel composition comprising a gas oil base fuel and an alkyl levulinate, which method comprises selecting by reference to its polar interaction parameter (δp) and hydrogen bonding parameter (δh) a co-solvent for which said polar interaction parameter is in the range of from 1 to 7 MPa 1/2 , preferably from 1.5 to 6.5 MPa 1/2 , more preferably from 2.5 to 6 MPa 1/2 , and said hydrogen bonding parameter is in the range of from 2 to 18 MPa 1/2 , preferably from 4 to 17 MPa 1/2 , more preferably from 6 to 16 MPa 1/2 , preferably an alcohol, and incorporating said selected co-solvent in the fuel composition.  
      In accordance with yet another embodiment of the present invention there is further provided use in a fuel composition comprising a gas oil base fuel and an alkyl levulinate of a co-solvent, preferably an alcohol, which co-solvent has a polar interaction parameter (δp) in the range of from 1 to 7 MPa 1/2 , preferably from 1.5 to 6.5 MPa 1/2 , more preferably from 2.5 to 6 MPa 1/2 , and a hydrogen bonding parameter (δh) in the range of from 2 to 18 MPa 1/2 , preferably from 4 to 17 MPa 1/2 , more preferably from 6 to 16 MPa 1/2 , for the purpose of reducing the phase separation temperature of the fuel composition.  
      In accordance with another embodiment of the present invention there is further provided a method of operating a compression ignition engine and/or a vehicle which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a fuel composition according to the present invention.  
      In accordance with yet another embodiment of the present invention there is further provided a method of operating a heating appliance provided with a burner, which method comprises supplying to said burner a fuel composition according to the present invention.  
      In accordance with yet another embodiment of the present invention there is further provided a process for the preparation of a fuel composition which process involves blending a gas oil base fuel, an alkyl levulinate and a co-solvent, preferably an alcohol, which co-solvent has a polar interaction parameter (δp) in the range of from 1 to 7 MPa 1/2 , preferably from 1.5 to 6.5 MPa 1/2 , more preferably from 2.5 to 6 MPa 1/2 , and a hydrogen bonding parameter (δh) in the range of from 2 to 18 MPa 1/2 , preferably from 4 to 17 MPa 1/2 , more preferably from 6 to 16 MPa 1/2 .  
      Preferably, said alkyl levulinate is selected from C 2-8  alkyl levulinates, preferably ethyl levulinate, n-propyl levulinate, n-butyl levulinate, n-pentyl levulinate, 2-hexyl levulinate, 2-ethyl hexyl levulinate and/or mixtures thereof, more preferably ethyl levulinate, n-butyl levulinate and/or n-pentyl levulinate, most preferably ethyl levulinate.  
      Preferably, said co-solvent is selected from C 3-8  alcohols, for example isopropanol, 1-butanol, isobutanol, 3-methyl-1-butanol, 1-pentanol, 2-butoxy-ethanol (i.e. butyl oxitol), 4-methyl-2-pentanol (i.e. methyl isobutyl carbinol), 2-ethyl hexanol, 2-[2-(1-butoxy)ethoxy]ethanol (i.e. butyl dioxitol) and mixtures thereof; hydrocarbons such as toluene; and oxygenates such as fatty acid alkyl esters, particularly rapeseed methyl ester (RME).  
      Preferably, the concentration of said co-solvents accords with one or more of the following parameters:  
      (i) at least 0.5% m; (ii) at least 1% m; (iii) at least 2% m; (iv) up to 5% m; (v) up to 8% m; (vi) up to 15% m, with ranges having features (i) and (vi), (ii) and (v), (iii) and (iv) respectively being progressively more preferred.  
      Preferably, said phase separation temperature is reduced by at least 3° C., more preferably by at least 5° C., still more preferably by at least 10° C., and most preferably by at least 20° C.  
      Preferably, said phase separation temperature of said fuel composition is below −5° C., more preferably below −10° C., still more preferably below −20° C., and most preferably below −30° C.  
      In all aspects of the present invention, blends of two or more of the alkyl levulinates may be included in the fuel composition. In the context of the present invention, selection of the particular components of said blends and their proportions is dependent upon one or more desired characteristics of the fuel composition.  
      The present invention may be used to formulate fuel blends which are expected to be of particular use in modern commercially available diesel engines as alternatives to the standard diesel base fuels, for instance as commercial and legislative pressures favour the use of increasing quantities of organically derived “biofuels”.  
      In the context of the present invention, “use” of a fuel component in a fuel composition means incorporating the component into the composition, typically as a blend (i.e. a physical mixture) with one or more other fuel components, conveniently before the composition is introduced into an engine.  
      The fuel composition will typically contain a major proportion of the base fuel, such as from 50 to 99% v, preferably from 50 to 98% v, more preferably from 80 to 98% v, most preferably from 90 to 98% v. The proportions of the alkyl levulinates and co-solvents will be chosen to achieve the desired degree of miscibility, i.e. phase separation temperature, and may also be influenced by other properties required of the overall composition.  
      The fuel compositions to which the present invention relates include diesel fuels for use in automotive compression ignition engines, as well as in other types of engine such as for example marine, railroad and stationary engines, and industrial gas oils for use in heating applications (e.g. boilers).  
      The base fuel may itself comprise a mixture of two or more different diesel fuel components, and/or be additivated as described below.  
      Such diesel fuels will contain a base fuel which may typically comprise liquid hydrocarbon middle distillate gas oil(s), for instance petroleum derived gas oils. Such fuels will typically have boiling points within the usual diesel range of 150 to 400° C., depending on grade and use. They will typically have a density from 750 to 900 kg/m 3 , preferably from 800 to 860 kg/m 3 , at 15° C. (e.g. ASTM D4502 or IP 365) and a cetane number (ASTM D613) of from 35 to 80, more preferably from 40 to 75. They will typically have an initial boiling point in the range 150 to 230° C. and a final boiling point in the range 290 to 400° C. Their kinematic viscosity at 40° C. (ASTM D445) might suitably be from 1.5 to 4.5 mm 2 /s.  
      Such industrial gas oils will contain a base fuel which may comprise fuel fractions such as the kerosene or gas oil fractions obtained in traditional refinery processes, which upgrade crude petroleum feedstock to useful products. Preferably such fractions contain components having carbon numbers in the range 5 to 40, more preferably 5 to 31, yet more preferably 6 to 25, most preferably 9 to 25, and such fractions have a density at 15° C. of 650 to 1000 kg/m 3 , a kinematic viscosity at 20° C. of 1 to 80 mm 2 /s, and a boiling range of 150 to 400° C.  
      Optionally, non-mineral oil based fuels, such as vegetable oil-based or animal fat-based biofuels or Fischer-Tropsch derived fuels, may also form or be present in the fuel composition. Such Fischer-Tropsch fuels may for example be derived from natural gas, natural gas liquids, petroleum or shale oil, petroleum or shale oil processing residues, coal or biomass.  
      The amount of Fischer-Tropsch derived fuel used in a diesel fuel composition may be from 0.5 to 100% v of the overall diesel fuel composition, preferably from 5 to 75% v. It may be desirable for the composition to contain 10% v or greater, more preferably 20% v or greater, still more preferably 30% v or greater, of the Fischer-Tropsch derived fuel. It is particularly preferred for the composition to contain 30 to 75% v, and particularly 30 or 70% v, of the Fischer-Tropsch derived fuel. The balance of the fuel composition is made up of one or more other fuels.  
      An industrial gas oil composition will preferably comprise more than 50 wt %, more preferably more than 70 wt %, of a Fischer-Tropsch derived fuel component.  
      Such a Fischer-Tropsch derived fuel component is any fraction of the middle distillate fuel range, which can be isolated from the (hydrocracked) Fischer-Tropsch synthesis product. Typical fractions will boil in the naphtha, kerosene or gas oil range. Preferably, a Fischer-Tropsch product boiling in the kerosene or gas oil range is used because these products are easier to handle in for example domestic environments. Such products will suitably comprise a fraction larger than 90 wt % which boils between 160 and 400° C., preferably to about 370° C. Examples of Fischer-Tropsch derived kerosene and gas oils are described in EP-A-0583836, WO-A-97/14768, WO-A-97/14769, WO-A-00/11116, WO-A-00/11117, WO-A-01/83406, WO-A-01/83648, WO-A-01/83647, WO-A-01/83641, WO-A-00/20535, WO-A-00/20534, EP-A-1101813, U.S. Pat. No. 5,766,274, U.S. Pat. No. 5,378,348, U.S. Pat. No. 5,888,376 and U.S. Pat. No. 6,204,426.  
      The Fischer-Tropsch product will suitably contain more than 80 wt % and more suitably more than 95 wt % iso and normal paraffins and less than 1 wt % aromatics, the balance being naphthenics compounds. The content of sulphur and nitrogen will be very low and normally below the detection limits for such compounds. For this reason the sulphur content of a fuel composition containing a Fischer-Tropsch product may be very low.  
      The fuel composition preferably contains no more than 5000 ppmw sulphur, more preferably no more than 500 ppmw, or no more than 350 ppmw, or no more than 150 ppmw, or no more than 100 ppmw, or no more than 50 ppmw, or most preferably no more than 10 ppmw sulphur.  
      In addition to the alkyl levulinates and the above-mentioned co-solvents, the fuel composition of the present invention may, if required, contain one or more additives as described below.  
      The base fuel may itself be additivated (additive-containing) or unadditivated (additive-free). If additivated, e.g. at the refinery, it will contain minor amounts of one or more additives selected for example from anti-static agents, pipeline drag reducers, flow improvers (e.g. ethylene/vinyl acetate copolymers or acrylate/maleic anhydride copolymers), lubricity additives, antioxidants and wax anti-settling agents.  
      Detergent-containing diesel fuel additives are known and commercially available. Such additives may be added to diesel fuels at levels intended to reduce, remove, or slow the build up of engine deposits.  
      Examples of detergents suitable for use in fuel additives for the present purpose include polyolefin substituted succinimides or succinamides of polyamines, for instance polyisobutylene succinimides or polyisobutylene amine succinamides, aliphatic amines, Mannich bases or amines and polyolefin (e.g. polyisobutylene) maleic anhydrides. Succinimide dispersant additives are described for example in GB-A-960493, EP-A-0147240, EP-A-0482253, EP-A-0613938, EP-A-0557516 and WO-A-98/42808. Particularly preferred are polyolefin substituted succinimides such as polyisobutylene succinimides.  
      The additive may contain other components in addition to the detergent. Examples are lubricity enhancers; dehazers, e.g. alkoxylated phenol formaldehyde polymers; anti-foaming agents (e.g. polyether-modified polysiloxanes); ignition improvers (cetane improvers) (e.g. 2-ethylhexyl nitrate (EHN), cyclohexyl nitrate, di-tert-butyl peroxide and those disclosed in U.S. Pat. No. 4,208,190 at column 2, line 27 to column 3, line 21); anti-rust agents (e.g. a propane-1,2-diol semi-ester of tetrapropenyl succinic acid, or polyhydric alcohol esters of a succinic acid derivative, the succinic acid derivative having on at least one of its alpha-carbon atoms an unsubstituted or substituted aliphatic hydrocarbon group containing from 20 to 500 carbon atoms, e.g. the pentaerythritol diester of polyisobutylene-substituted succinic acid); corrosion inhibitors; reodorants; anti-wear additives; anti-oxidants (e.g. phenolics such as 2,6-di-tert-butylphenol, or phenylenediamines such as N,N′-di-sec-butyl-p-phenylenediamine); metal deactivators; and combustion improvers.  
      It is particularly preferred that the additive include a lubricity enhancer, especially when the fuel composition has a low (e.g. 500 ppmw or less) sulphur content. In the additivated fuel composition, the lubricity enhancer is conveniently present at a concentration of less than 1000 ppmw, preferably between 50 and 1000 ppmw, more preferably between 100 and 1000 ppmw. Suitable commercially available lubricity enhancers include ester- and acid-based additives. Other lubricity enhancers are described in the patent literature, in particular in connection with their use in low sulphur content diesel fuels, for example in: 
          the paper by Danping Wei and H. A. Spikes, “ The Lubricity of Diesel Fuels ”, Wear, III (1986) 217-235;     WO-A-95/33805—cold flow improvers to enhance lubricity of low sulphur fuels;     WO-A-94/17160—certain esters of a carboxylic acid and an alcohol wherein the acid has from 2 to 50 carbon atoms and the alcohol has 1 or more carbon atoms, particularly glycerol monooleate and di-isodecyl adipate, as fuel additives for wear reduction in a diesel engine injection system;     U.S. Pat. No. 5,490,864—certain dithiophosphoric diester-dialcohols as anti-wear lubricity additives for low sulphur diesel fuels; and     WO-A-98/01516—certain alkyl aromatic compounds having at least one carboxyl group attached to their aromatic nuclei, to confer anti-wear lubricity effects particularly in low sulphur diesel fuels.        

      It is also preferred that the additive contain an anti-foaming agent, more preferably in combination with an anti-rust agent and/or a corrosion inhibitor and/or a lubricity additive.  
      Unless otherwise stated, the (active matter) concentration of each such additional component in the additivated fuel composition is preferably up to 10000 ppmw, more preferably in the range from 0.1 to 1000 ppmw, advantageously from 0.1 to 300 ppmw, such as from 0.1 to 150 ppmw.  
      The (active matter) concentration of any dehazer in the fuel composition will preferably be in the range from 0.1 to 20 ppmw, more preferably from 1 to 15 ppmw, still more preferably from 1 to 10 ppmw, advantageously from 1 to 5 ppmw. The (active matter) concentration of any ignition improver present will preferably be 2600 ppmw or less, more preferably 2000 ppmw or less, conveniently from 300 to 1500 ppmw.  
      If desired, the additive components, as listed above, may be co-mixed, preferably together with suitable diluent(s), in an additive concentrate, and the additive concentrate may be dispersed into the fuel, in suitable quantity to result in a composition of the present invention.  
      In the case of a diesel fuel composition, for example, the additive will typically contain a detergent, optionally together with other components as described above, and a diesel fuel-compatible diluent, which may be a carrier oil (e.g. a mineral oil), a polyether, which may be capped or uncapped, a non-polar solvent such as toluene, xylene, white spirits and those sold by Shell companies under the trade mark “SHELLSOL”, and/or a polar solvent such as an ester and, in particular, an alcohol, e.g. hexanol, 2-ethylhexanol, decanol, isotridecanol and alcohol mixtures such as those sold by Shell companies under the trade mark “LINEVOL”, especially LINEVOL 79 alcohol which is a mixture of C 7-9  primary alcohols, or a C 12-14  alcohol mixture which is commercially available.  
      The total content of the additives may be suitably between 0 and 10000 ppmw and preferably below 5000 ppmw.  
      Preferably, the alkyl levulinate concentration in the fuel composition accords with one or more of the following parameters:  
      (i) at least 1% v; (ii) at least 2% v; (iii) at least 3% v; (iv) at least 4% v; (v) at least 5% v; (vi) up to 6% v; (vii) up to 8% v; (viii) up to 10% v, (xi) up to 12% v, (x) up to 35% v, with ranges having features (i) and (x), (ii) and (ix), (iii) and (viii), (iv) and (vii), and (v) and (vi) respectively being progressively more preferred.  
      In this specification, amounts (concentrations, % v, ppmw, wt %) of components are of active matter, i.e. exclusive of volatile solvents/diluent materials.  
      The present invention is particularly applicable where the fuel composition is used or intended to be used in a direct injection diesel engine, for example of the rotary pump, in-line pump, unit pump, electronic unit injector or common rail type, or in an indirect injection diesel engine. The fuel composition may be suitable for use in heavy and/or light duty diesel engines.  
      As mentioned above, it is also applicable where the fuel composition is used in heating applications, for example boilers. Such boilers include standard boilers, low temperature boilers and condensing boilers, and are typically used for heating water for commercial or domestic applications such as space heating and water heating.  
      The present invention may lead to any of a number of advantageous effects, including good engine low temperature performance.  
      The present invention will now be further described by reference to the following Examples, in which, unless otherwise indicated, parts and percentages are by weight, and temperatures are in degrees Celsius:  
      Fuels were blended with additives by adding said additives to base fuel at ambient temperature (20° C.) and homogenising.  
      The following additives were used: 
      (a) ethyl levulinate (available ex. Aldrich);     (b) 1-pentanol (δp=4.5 MPa 1/2 ; δh=13.9 MPa 1/2 ) (available ex. Aldrich);     (c) 2-ethyl hexanol (δp=3.3 MPa 1/2 ; δh=11.9 MPa 1/2 ) (available ex. Aldrich);     (d) 3-methyl-1-butanol (δp=5.2 MPa 1/2 ; δh=13.4 MPa 1/2 ) (available ex. Aldrich);     (e) 4-methyl-2-pentanol (δp=3.3 MPa 1/2 ; δh=12.3 MPa 1/2 ) (available ex. Aldrich);     (f) 2-butoxy ethanol (δp=6.3 MPa 1/2 ; δh=12.9 MPa 1/2 ) (available ex. Aldrich);     (g) 2-[2-(1-butoxy)ethoxy]ethanol (δp=6.6 MPa 1/2 ; δh=11.9 MPa 1/2 ) (available ex. Shell Chemicals);     (h) 2-methyl-2,4-pentanediol, i.e. hexylene glycol (δp=8.4 MPa 1/2 ; δh=17.8 MPa 1/2 ) (available ex. Aldrich);     (i) toluene (δp=1.4 MPa 1/2 ; δh=2.0 MPa 1/2 ) (available ex. Aldrich);     (j) rapeseed methyl ester (for soybean methyl ester, δp=4.9 MPa 1/2 ; δh=5.9 MPa 1/2 ) (available ex. Diester Industrie); and     (k) tetrahydrofurfuryl alcohol (δp=8.7 MPa 1/2 ; δh=15.0 MPa 1/2 ) (available ex Aldrich).    

     EXAMPLES  
     Example 1  
      The miscibility of levulinates depends to some extent on base fuel properties. Two base fuels representative of the European market were chosen to explore this effect, i.e. (1) Fuel A was a Dreyfuss ULSD, a hydrotreated AGO having a cloud point of −27° C. and an aromatics content of 22% m; and (2) Fuel B was a Swedish Class 1 AGO, which is a low density, low aromatics (4% m) diesel fuel with a cloud point of −38° C. Both base fuels met the EN590 specification.  
      The properties of Fuels A and B are given in Table 1:  
                           TABLE 1                                   Fuel A   Fuel B                                                        Density @ 15° C., kg/m 3     822   815           Distillation T50, ° C.   242   235           Distillation T95, ° C.   304   272           Cetane Number   54   54           Viscosity @40° C., mm 2 /s   2.10   2.03           Sulphur, mg/kg   10   &lt;5           Cloud Point, ° C.   −27   −38           Aromatics, % m   22   4                      
 
      For screening purposes, a simple test method was used to determine the room temperature (20° C.) limit of miscibility of ethyl levulinate. Accurately metered volumes of ester were added sequentially to a known volume of diesel fuel in a 15 ml glass vial, shaken and observed. The first appearance of haze was recorded as the room temperature limit of miscibility for the mixture. The results are shown in Table 2 and clearly show that Fuel A solubilised more ethyl levulinate than Fuel B.  
                           TABLE 2                                   Fuel A   Fuel B                          14% v   7% v                      
 
      The miscibility of the ethyl levulinate was measured using a method based on the ASTM D2500 “Cloud Point” procedure. In this procedure, a sample of fuel (40 ml) is cooled from ambient temperature (20° C.) in a series of thermostat baths maintained at progressively lower temperatures. The sample is examined at 1° C. intervals as it cools to its wax cloud point. In addition to the wax cloud point temperature described in ASTM D2500, a further two temperatures were recorded coinciding with the following observations, if they occurred:  
      (1) the appearance of the first haze,  
      (2) the first sign of dropout of a separate liquid phase. In each case, cooling continued to the wax cloud point—beyond which, no further phase separation could be observed reliably, because the sample became opaque.  
      Solutions of the ester ethyl levulinate in Fuel A were blended at various concentrations and the miscibility of each blend was measured. The results are shown in Table 3 below:  
                           TABLE 3                                       Phase separation           Ester concentration (% v)   temperature (° C.)                                                    2   W           3   W           4   −17           5   −10           6    −5*           8    7           10    14                         W denotes that the mixture was cooled to the wax cloud point (−27° C. for Fuel A) without liquid separation;                *= extrapolated value             
 
      The miscibility tests were repeated using Fuel B. The results are shown in Table 4:  
                           TABLE 4                                       Phase separation           Ester concentration (% v)   temperature (° C.)                          2   −26           3   −10           4    3           5    5           6    10*                         *= extrapolated value             
 
      It has been found that, when various quantities of co-solvent alcohols were added to Fuels A and B containing 5% vol ethyl levulinate, this had the effect of reducing the phase separation temperature by various amounts. The results of tests which demonstrate this effect in respect of Fuel A are set out in Table 5:  
                           TABLE 5                               Co-solvent           Co-solvent       2-ethyl       1-pentanol   Phase separation   hexanol   Phase separation       (% v)   temperature (° C.)   (% v)   temperature (° C.)                  0   −10    0   −10       1   −19*   1    −17*       2   −28*   2   −24       3   −28*   3   —       4   −27*   4   —       5   −27*   5   —       6   −27*   6   —                 *= extrapolated value             
 
      The results of tests which demonstrate this effect in respect of Fuel B are set out in Table 6:  
                           TABLE 6                                       Phase separation           Co-solvent 2-ethyl hexanol (% v)   temperature (° C.)                          0    5           1    1*           2   −3           3    −7*           4   −12*           5   −15            6   −19*                         *= extrapolated value             
 
     Example 2  
      The miscibility of ethyl levulinate was measured in n-decane as a model diesel component using 1-pentanol and 2-ethyl hexanol as co-solvents. The results are shown in Table 7:  
                       TABLE 7                                      Phase separation temperature (° C.)                             No                                 1-pentanol           Ester   and no                             concentration   2-ethyl   1-pentanol   2-ethyl hexanol                                     (% w)   hexanol   2% w   4.8% w   2% w   4.8% w                                             3   −20    &lt;−20     &lt;−26     &lt;−20    &lt;−25         5   −3    −14    −26    −16    −25        6   0   −12*    −19*    −10    −20*        8    8*   0   −10     0   −12*        10   16*    3*   0    6*   −8*       15   28    13    5   20   4       20   34*   23*   14*    27*   12*       25   38*   27*   17*    32*   17*       30   41    31    20    33   20        35   42*   32*   22*   34   23*                 *= extrapolated value             
 
      It can be seen quite clearly from the figures in Table 7 that each of the alcohol co-solvents 1-pentanol and 2-ethyl hexanol had the effect of reducing the phase temperature with a broad range of concentrations of ethyl levulinate in n-decane.  
      It has been found that a number of further co-solvents, when added to n-decane containing 5% w ethyl levulinate, also had the effect of reducing the phase separation temperature, which was −3° C. before addition of said co-solvents. This can be seen from the results set out in Table 8, which show the phase separation temperatures when the co-solvents are added in the amounts of 2% w and 4.8% w:  
                           TABLE 8                                   Co-solvent   Phase separation           (% w)   temperature (° C.)                                                1-pentanol   2   −14           4.8   −26       2-ethyl hexanol   2   −16           4.8   −24       3-methyl-1-butanol   2   −15           4.8   −21       4-methyl-2-pentanol   2   −18           4.8   −24       2-butoxy ethanol   2   −10           4.8   −18       *2-[2-(1-butoxy)ethoxy] ethanol   2   −6           4.8   −9       Toluene   2   −6           4.8   −10       rapeseed methyl ester   2   n.d.           4.8   −8                 n.d. not determined             
 
      It can be seen quite clearly from the figures in Table 8 that each of the co-solvents listed, which exhibit values of polar interaction parameter (δp) and hydrogen bonding parameter (δh) within the respective ranges specified above, had the effect of reducing the phase separation temperature of the composition containing n-decane and 5% w ethyl levulinate.  
      By way of comparison, it has also been shown that co-solvents which exhibit values of polar interaction parameter (δp) and hydrogen bonding parameter (δh) outside the respective ranges specified above, do not have the effect of reducing the phase separation temperature of the composition containing n-decane and 5% w ethyl levulinate.  
      This can be seen from the results set out in Table 9, which show the phase separation temperatures when such co-solvents are added in the amounts of 2% w and 4.8% w:  
                           TABLE 9                                   Co-solvent   Phase separation           (% w)   temperature (° C.)                                                        2-methyl-2,4-pentanediol   2   −2               4.8   4           tetrahydrofurfuryl alcohol   2   13               4.8   18