WO2009068539A2

WO2009068539A2 - Gasoline compositions

Info

Publication number: WO2009068539A2
Application number: PCT/EP2008/066172
Authority: WO
Inventors: Christopher William Clayton; Richard John Price; Nigel Peter Tait; Robert Ian Taylor
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2007-11-28
Filing date: 2008-11-25
Publication date: 2009-06-04
Also published as: US20090188156A1; WO2009068539A3; AR069738A1

Abstract

The present invention provides a gasoline composition comprising: (a) a major amount of gasoline; and (b) from 0.1 to 5 wt%, based on the overall gasoline composition, of a base oil having a kinematic viscosity at 100 0C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process. The present invention further provides a process for preparing a gasoline composition and a method of operating a four-stroke spark-ignition internal combustion engine.

Description

GASOLINE COMPOSITIONS

Field of the Invention

The present invention provides a gasoline composition suitable for use in a spark-ignition (SI) internal combustion engine. Background of the Invention

The use of base oils as carrier fluids for fuel additives is disclosed in the art.

WO 97/41092 Al (Shell) discloses alkoxy acetic acid derivatives and their incorporation in additive concentrates and fuel compositions for internal combustion engines. The carrier fluids disclosed in WO 97/41092 Al include hydrocarbon based materials such as polyisobutylenes (PIB' s), polypropylenes (PP' s) and polyalphaolefins (PAO' s) , all of which may be hydrogenated or unhydrogenated but are preferably hydrogenated; polyether based materials such as polybutylene oxides (poly BO' s), polypropylene oxides (poly PO' s) , polyhexadecene oxides (poly HO' s) and mixtures thereof (i.e. both (poly BO)+ (poly PO) and poly- BO-PO) ) ; and mineral oils such as those sold by member companies of the Royal Dutch/Shell group under the designations "HVI" and "XHVI" (trade mark), Exxon Naphthenic 900 sus mineral oil and high viscosity index oils in general. A particularly prepared carrier fluid disclosed in

WO 97/41092 Al comprises a combination of a polyalphaolefin having a viscosity at 100 ⁰C in the range

2^χlO^~6 to 2xlO^~5 m²/s (2 to 20 cSt) being a hydrogenated oligomer containing 18 to 80 carbon atoms derived from at least one alphaolefinic monomer containing from 8 to 16 carbon atoms, and a polyoxyalkylene compound selected from glycols, mono- and diethers thereof, having number average molecular weight (M_n) in the range 400 to 3000, the weight ratio polyalphaolefin: polyoxyalkylene compound being in the range 1:10 to 10:1.

The carrier concentration in the final fuel composition of WO 97/41092 Al is described as up to about 1000 ppm weight, with a stated preference for concentrations of from about 50 ppm by weight to about 400 ppm by weight, based on the total weight of the fuel composition.

WO 97/41092 Al does not disclose any beneficial effects, in particularly in terms of improved acceleration response, associated with any of the carrier fluids disclosed therein.

WO 03/068895 Al (Shell) discloses a gasoline composition comprising a major amount of a gasoline suitable for use in a spark ignition engine; 5 to 1000 ppmw, based on total composition, of a fatty acid salt of an alkoxylated oligoamine of general formula I (AO)_x-H [ _κ-< _Jm+l (I)

wherein each moiety A independently represents an alkylene group of 2 to 8 carbon atoms, each moiety R independently represents a C7_23 alkyl or singly or multiply-unsaturated C7 to C23 alkenyl group, optionally substituted by one or more -OH groups, each moiety Z independently represents a C]__g alkylene group, a C3--8 cycloalkylene group, or a C§™]_2 arylene or arylalkylene group, m represents 0 or an integer in the range 1 to 5, and the total of all variables x has a value of 50% to 300% of (m+3); and 600 to 2000 ppmw, based on total composition, of a co-additive selected from the group consisting of (a) a nitrogen-containing detergent containing a hydrocarbyl group having a number average molecular weight (Mn) in the range 750 to 6000, (b) a polyalphaolefin having a viscosity at 100⁰C in the range

2xlO^"6 to 2xlO^~5 m²/s (2 to 20 centistokes) , being an oligomer containing 18 to 80 carbon atoms derived from at least one alphaolefinic monomer containing from 8 to 16 carbon atoms; (c) a polyoxyalkylene compound selected from glycols, mono- and diethers thereof, having number average molecular weight (Mn) in the range 400 to 3000; and mixtures of any two or all three of (a) , (b) and (c) . The fatty acid salt of an alkoxylated oligoamine incorporated in the gasoline composition disclosed in WO 03/068895 Al is described as a friction modifier. WO 03/068895 Al discloses enhanced acceleration performance of a spark-ignition engine from the use of fatty acid salt of an alkoxylated oligoamine in combination with a co-additive incorporated in a gasoline composition, however, WO 03/068895 Al does not disclose any beneficial effects, in particularly in terms of improved acceleration response, associated with the use of any of the co-additives disclosed therein. It has now been found that the use of base oils having a kinematic viscosity at 100 ⁰C of at least 1 cSt, wherein at least 90 %v/v of the base oil composition is derived from the product of a Fischer-Tropsch synthesis process, in admixture with a gasoline provides a gasoline composition which surprisingly exhibits an improvement in the acceleration response of a four-stroke spark-ignition internal combustion engine. Summary of the Invention

The present invention provides a gasoline composition comprising:

(a) a major amount of gasoline; and (b) from 0.1 to 5 wt%, based on the overall gasoline composition, of a base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process. The present invention further provides a process for preparing a gasoline composition, comprising bringing into admixture with gasoline, from 0.1 wt% to 5 wt%, based on the overall gasoline composition, of a base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process.

The present invention yet further provides a method of operating a four-stroke spark-ignition internal combustion engine, which method involves introducing into a combustion chamber of said engine a gasoline composition according to the present invention. Detailed Description of the Invention

The gasoline composition of the present invention comprises: (a) a major amount of gasoline; and

(b) from 0.1 to 5 wt%, based on the overall gasoline composition, of a base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process.

The gasoline may be any gasoline suitable for use in an internal combustion engine of the spark-ignition (petrol) type known in the art. The gasoline used in the ~~ O ~~ gasoline composition of the present invention {i.e. component (a) above) may conveniently also be referred to as ^Λbase gasoline' .

Gasolines typically comprise mixtures of hydrocarbons boiling in the range from 25 to 230 ⁰C (EN- ISO 3405) , the optimal ranges and distillation curves typically varying according to climate and season of the year. The hydrocarbons in a gasoline may be derived by any means known in the art, conveniently the hydrocarbons may be derived in any known manner from straight-run gasoline, synthetically-produced aromatic hydrocarbon mixtures, thermally or catalytically cracked hydrocarbons, hydro-cracked petroleum fractions, catalytically reformed hydrocarbons or mixtures of these. The specific distillation curve, hydrocarbon composition, research octane number {RON) and motor octane number (MON) of the gasoline are not critical.

Conveniently, the research octane number (RON) of the gasoline may be in the range of from 80 to 110, preferably from 90 to 105, more preferably from 93 to

102, most preferably from 94 to 100 (EN 25164); the motor octane number (MON) of the gasoline may suitably be in the range of from 70 to 110, preferably from 75 to 105, more preferably from 80 to 100, most preferably from 84 to 95 (EN 25163} .

Typically, gasolines comprise components selected from one or more of the following groups; saturated hydrocarbons, olefinic hydrocarbons, aromatic hydrocarbons, and oxygenated hydrocarbons. Conveniently, the gasoline may comprise a mixture of saturated hydrocarbons, olefinic hydrocarbons, aromatic hydrocarbons, and, optionally, oxygenated hydrocarbons. Typically, the olefinic hydrocarbon content of the gasoline is in the range of from 0 to 40 percent by volume based on the gasoline; preferably, the olefinic hydrocarbon content of the gasoline is in the range of from 0 to 30 percent by volume based on the gasoline.

Typically, the aromatic hydrocarbon content of the gasoline is in the range of from 0 to 70 percent by volume based on the gasoline; preferably, the aromatic hydrocarbon content of the gasoline is in the range of from 10 to 60 percent by volume based on the gasoline. The benzene content of the gasoline is at most 10 percent by volume, more preferably at most 5 percent by volume, especially at most 1 percent by volume based on the gasoline. Typically, the saturated hydrocarbon content of the gasoline is at least 40 percent by volume based on the gasoline; preferably, the saturated hydrocarbon content of the gasoline is in the range of from 40 to 80 percent by volume based on the gasoline. The gasoline preferably has a low or ultra low sulphur content, for instance at most 1000 ppmw (parts per million by weight) , preferably no more than 500 ppmw, more preferably no more than 100, even more preferably no more than 50 and most preferably no more than even 10 ppmw .

The gasoline also preferably has a low total lead content, such as at most 0.005 g/1, most preferably being lead free - having no lead compounds added thereto (i.e. unleaded) • When the gasoline comprises oxygenated hydrocarbons, at least a portion of non-oxygenated hydrocarbons will be substituted for oxygenated hydrocarbons. When gasoline contains oxygenated hydrocarbons, the oxygen content of the gasoline may be up to 35 percent by weight (e.g. ethanol per se) based on the gasoline. For example, the oxygen content of the gasoline may be up to 25 percent by weight, preferably up to 10 percent by weight. Examples of suitable ranges of oxygen content of the gasoline are from 0 to 5 percent by weight, preferably from 1.0 to 3.5 percent by weight, and more preferably from 1.2 to 2.7 percent by weight. Examples of oxygenated hydrocarbons that may be incorporated into gasolines include alcohols, ethers, esters, ketones, aldehydes, carboxylic acids and their derivatives, and oxygen containing heterocyclic compounds. Preferably, the oxygenated hydrocarbons that may be incorporated into gasolines are selected from alcohols (such as methanol, ethanol, propanol, iso- propanol, butanol, tert-butanol and iso-butanol) and ethers (preferably ethers containing 5 or more carbon atoms per molecule, e.g., methyl tert-butyl ether), a particularly preferred oxygenated hydrocarbon is ethanol.

The amount of oxygenated hydrocarbons in gasoline may vary over a wide range. For example, gasolines comprising a major proportion of oxygenated hydrocarbons are currently commercially available in countries such as Brazil and U. S.A, e.g. ethanol per se and E85, as well as gasolines comprising a minor proportion of oxygenated hydrocarbons, e.g. ElO. Gasolines may contain up to 100 percent by volume oxygenated hydrocarbons. Preferably, the amount of oxygenated hydrocarbons present in gasoline is selected from one of the following amounts: up to 85 percent by volume; up to 65 percent by volume; up to 30 percent by volume; up to 20 percent by volume; up to 15 percent by volume; and, up to 10 percent by volume, _ Q

depending upon the desired final formulation of the gasoline. Conveniently, the gasoline may contain at least 0.5, 1.0 or 2.0 percent by volume oxygenated hydrocarbons . Examples of suitable gasolines include gasolines which have an olefinic hydrocarbon content of from 0 to 20 percent by volume (ASTM D1319) , an oxygen content of from 0 to 5 percent by weight (EN 1601) , an aromatic hydrocarbon content of from 0 to 50 percent by volume (ASTM D1319) and a benzene content of at most 1 percent by volume.

The base oil admixed with the gasoline in the gasoline composition of the present invention has a kinematic viscosity at 100 ⁰C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process .

Preferably, the kinematic viscosity at 100 ⁰C of the base oil admixed with the gasoline in the present invention accords with one of parameters (i) to (ix) below, or a combination of one of parameters (i) to (v) and one of parameters {vi) to {ix) :- (i) at least 4 cSt, {ii) at least 8 cSt, (iii) at least 10 cSt, (iv) at least 12 cSt,

(v) at least 16 cSt, with features (i) , (ii) , (iii) , (iv) and (v) being progressively more preferred; and

(vi) up to 60 cSt, (vii) up to 50 cSt,

(viii) up to 40 cSt,

(ix) up to 35 cSt, with features (vi) , (vii) , (viii) and (ix) being progressively more preferred.

Ranges having a combination of any feature selected from (i) through (v) above and any feature selected from (vi) through (ix) above are particularly applicable in the present invention. Examples of specific combinations of the above features include (i) and {vi) , (ii) and (vii) , (iii) and (viii) , (iv) and (ix) , and (v) and (ix) , respectively being progressively more preferred. Typically, the base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process will have a viscosity index (VI) (ASTM D 2270) of at least 80. More typically, the base oil having a kinematic viscosity at 100 °C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process will have a VI of at least 90, conveniently the base oil will have a VI of at least 95. In particular, the VI of the base oil may be at least 100, for instance at least 110 or even at least 120. Typically, the VI of the base oil will be at most 300, more typically at most 250; conveniently, the VI of the base oil may be at most 200.

The boiling point of the base oil is not critical, however, typically the boiling point will be at least 260 ⁰C, more typically at least 300 ⁰C.

In addition to the base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process, the gasoline composition of the present invention may also include one or more non-Fischer-Tropsch derived base oil. Conveniently, conventional known mineral oils and synthetic oils or mixtures of one or more mineral oils and/or one or more synthetic oils may be included in the gasoline composition in addition to the base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process . Mineral oils include liquid petroleum oils and solvent-treated or acid-treated mineral lubricating oil of the paraffinic, naphthenic, or mixed paraffinic/naphthenic type which may be further refined by hydrofinishing processes and/or dewaxing. Naphthenic base oils have low viscosity index (VI)

(generally 40-80} and a low pour point. Such base oils are produced from feedstocks rich in naphthenes and low in wax content.

Paraffinic base oils have higher VI (generally >95) and a high pour point. Said base oils are produced from feedstocks rich in paraffins.

Synthetic processes enable molecules to be built from simpler substances or to have their structures modified to give the precise properties required. Synthetic base oils include hydrocarbon oils such as olefin oligomers, dibasic acid esters, polyol esters, and dewaxed waxy raffinate. An example of synthetic base oils that may be used are the synthetic hydrocarbon base oils sold by the Shell Group under the designation "XHVI" (trade mark) .

If a non-Fischer-Tropsch derived base oil or mixture of non-Fischer-Tropsch derived base oils is included in the gasoline composition of the present invention, preferably this base oil is a liquid hydrocarbon base oil, such as a base oil selected from a Group I to Group IV base stock, or mixture thereof, having a kinematic viscosity at 100 ⁰C of at least 1 cSt. Conveniently, this may be a liquid hydrocarbon base oil derived from a mineral oil and/or a synthetically derived liquid hydrocarbon base oil, such as polyalphaolefin (PAO) compositions and/or polyinternalolefin (PIO) compositions . The base oil that is derived from the product of a

Fischer-Tropsch synthesis process may comprise a single base oil or mixture of base oils derived from the product of a Fischer-Tropsch synthesis process. Non-limiting examples of Fischer-Tropsch derived base oils are disclosed in EP-A-0776959, EP-A-0668342, WO-A-97/21788, WO-A-00/15736, WO-A-00/14188, WO-A-00/14187, WO-A- 00/14183, WO-A-00/14179, WO-A-00/08115, WO-A-99/41332, EP-A-1029029, WO-A-01/18156 and WO-A-01/57166.

The Fischer-Tropsch synthesis process converts carbon monoxide and hydrogen into longer chain, usually paraffinic, hydrocarbons: n(C0 + 2H₂) → (-CH₂-) _n + ^nH2° + heat, in the presence of an appropriate catalyst and typically at elevated temperatures (eg, 125 to 300 ⁰C, preferably 175 to 250 ⁰C) and/or pressures (eg, 5 to 100 bar, preferably 12 to 50 bar) . Hydrogen: carbon monoxide ratios other than 2:1 may be employed if desired.

The carbon monoxide and hydrogen may themselves be derived from organic or inorganic, natural or synthetic sources, typically either from natural gas or from organically derived methane. The gases which are converted into liquid fuel components using such processes can in general include natural gas (methane) , LPG (e.g. propane or butane), "condensates" such as ethane, synthesis gas (CO/hydrogen) and gaseous products derived from coal, biomass and other hydrocarbons.

Typical catalysts for the Fischer-Tropsch synthesis of paraffinic hydrocarbons comprise, as the catalytically active component, a metal from Group VIII of the periodic table, in particular ruthenium, iron, cobalt or nickel. Suitable such catalysts are described for instance in EP-A-O 583 836 (pages 3 and 4) . The Fischer-Tropsch synthesis process from which the

Fischer-Tropsch derived base oil is derived from is not critical and any Fischer-Tropsch synthesis process known in the art may be used. Examples of Fischer-Tropsch synthesis processes which can be used to prepare the above-described Fischer-Tropsch derived base oil are the so-called commercial Slurry Phase Distillate technology of Sasol, the Shell Middle Distillate Synthesis Process (SMDS) and the ^XXAGC~21" Exxon Mobil process. These and other processes are, for example, described in more detail in EP-A-776 959, EP-A-668 342, US-A-4 943 672, US-A-5 059 299, WO-A-99/34917 and WO-A-99/20720. Typically these Fischer-Tropsch synthesis products will comprise hydrocarbons having 1 to 100 and even more than 100 carbon atoms. The product of the Fischer-Tropsch synthesis process will contain saturated hydrocarbon products (normal, cyclic and/or iso-paraffins) and optionally may also comprise unsaturated hydrocarbon products and oxygenated hydrocarbon products, depending upon the specific Fischer-Tropsch process used. The Fischer-Tropsch derived base oil may be obtained directly from the Fischer-Tropsch synthesis process, or indirectly for instance by fractionation of a Fischer-Tropsch synthesis product or from a treated Fischer-Tropsch synthesis product. The treatment that may be applied to the Fischer-Tropsch synthesis product can be any hydrocarbon treatment known in the art. Examples of suitable treatments that may be performed on the Fischer-Tropsch synthesis product include dewaxing treatments, a clay treating process or contacting with active carbon such as described in US-A-4795546 and EP-A-712922, hydrotreating, hydrocracking, hydrofinishing, hydrogenation, hydroisomerization, polymerisation, alkylation, distillation, cracking-decarboxylation, isomerisation and hydroreforming, as described for instance in GB-2 077 289 B, EP-A-O 147 873, US-A-4, 125, 566 and US-A-4, 478, 955. Conveniently, the base oil derived from a Fischer-Tropsch synthesis process may be a base oil derived from the processes described in WO 2004/007647 Al and US 2005/0098476 Al.

By virtue of the Fischer-Tropsch synthesis process, a base oil derived from a Fischer-Tropsch synthesis process has essentially no (i.e. undetectable), or very low levels of, sulphur and nitrogen. Compounds containing these heteroatoms tend to act as poisons for Fischer-Tropsch catalysts and are therefore removed from the synthesis gas feed. Further, the process as usually operated produces no or very low levels of aromatic compounds .

The concentration of the base oil admixed with the gasoline in the present invention will be in the range of from 0.1 to 5 wt%, based on the overall gasoline composition.

Preferably, the concentration, based on the overall gasoline composition, of the base oil admixed with the gasoline in the present invention accords with one of parameters (x) to (xix) below, or a combination of one of parameters (x) to (xiv) and one of parameters (xv) to (xix) : -

(x) at least 0.2 wt%; (xi) greater than 0.2 wt%; (xii ) at least 0 . 25 wt% ;

(xiii ) at least 0 . 3 wt% ;

(xiv) at least 0 . 35 wt% ; with features (x) , (xi) , (x±i) , (xiii) and (xiv) being progressively more preferred; and

(xv) up to 4.5 wt%;

(xvi) up to 4.0 wt%;

(xvii) up to 3.5 wt%;

{xviii) up to 3.0 wt%; (xix) up to 2.5 wt%; with features (xv) , (xvi) , {xvii) , (xviii) and (xix) being progressively more preferred.

Ranges having a combination of any feature selected from (x) through (xiv) above and any feature selected from (xv) through {xix) above are particularly applicable in the present invention. Examples of specific combinations of the above features include (x) and (xv) , (xi) and (xvi), (xii) and (xvii), {xiii) and {xviii), and (xiv) and {xix) , respectively being progressively more preferred.

In one embodiment of the present invention, the concentration, based on the overall gasoline composition, of the base oil admixed with the gasoline in the present invention may be at least 1.0 wt%. For example the concentration of the base oil admixed with the gasoline may be in the range of from 1.0 to 5.0 wt%, from 1.0 to 4.5 wt%, from 1.0 to 4.0 wt%, from 1.0 to 3.5 wt%, from 1.0 to 3.0 wt% or even from 1.0 to 2.5 wt%.

Whilst not critical to the present invention, the gasoline composition may conveniently additionally include one or more fuel additive. The concentration and nature of the fuel additive (s) that may be included in the gasoline composition of the present invention is not critical. Non-limiting examples of suitable types of fuel additives that can be included in the gasoline composition include anti-oxidants, corrosion inhibitors, detergents, dehazers, antiknock additives, metal deactivators, valve-seat recession protectant compounds, dyes, friction modifiers, carrier fluids, diluents and markers. Examples of suitable such additives are described generally in US Patent No. 5,855,629.

Conveniently, the fuel additives can be blended with one or more diluents or carrier fluids, to form an additive concentrate, the additive concentrate can then be admixed with the gasoline. If the one or more diluents or carrier fluids used is a base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt, the concentration of the base oil admixed with the gasoline to form the gasoline composition will be adjusted accordingly.

The (active matter) concentration of any additives present in the gasoline or the gasoline composition is preferably up to 1 percent by weight, more preferably in the range from 5 to 1000 ppmw, advantageously in the range of from 75 to 300 ppmw, such as from 95 to 150 ppmw.

A gasoline composition according to the present invention may be prepared by a process which comprises bringing into admixture with gasoline, from 0.1 wt% to 5 wt%, based on the overall gasoline composition, of a base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process, and optionally other conventional gasoline components, such as one or more fuel additives.

If desired, when one or more fuel additives are to be incorporated into the gasoline composition of the present invention, said fuel additives may be blended with the base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt to form an additive concentrate containing the fuel additives at suitable concentrations to result in a gasoline composition having the desired concentration of the fuel additives when the desired quantity of base oil chosen from 0.1 wt% to 5 wt%, based on the overall gasoline composition, of this additive concentrate is admixed with gasoline. It has been observed that the use of gasoline compositions according to the present invention in a four-stroke spark-ignition internal combustion engine has provided an improvement in the acceleration response of the engine. By the term "improvement in the acceleration response", it is meant that a four-stroke spark-ignition internal combustion engine fuelled with a gasoline composition according to the present invention takes less time to accelerate between two specific speeds {as determined by the rpm of the engine) than the same engine, operating under the same conditions, fuelled using a gasoline composition comprising only the base gasoline {i.e. not containing a base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt) .

Thus, the present invention provides the use of from 0.1 to 5 wt%, based on overall gasoline composition, of a base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process, in a gasoline composition comprising a major amount of gasoline, for improving the acceleration response of a four-stroke spark-ignition internal combustion engine fuelled by the gasoline composition. _ i 7 _™

The present invention also encompasses the use of a gasoline composition comprising:

(a) a major amount of a gasoline; and

(b) from 0.1 wt% to 5 wt%, based on the overall gasoline composition, of a base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt which is derived from the product of a Fischer-Tropsch synthesis process, for providing an improvement in the acceleration response of a four-stroke spark-ignition internal combustion engine fuelled by the gasoline composition.

The present invention also provides a method of operating a four-stroke spark-ignition internal combustion engine, which comprises bringing into the combustion chambers of said engine a gasoline composition as defined above.

The present invention will be further understood from the following examples, which illustrate the effects of gasoline compositions according to the present invention on the acceleration performance of a four-stroke spark-ignition internal combustion engine. Unless otherwise indicated, parts and percentages (concentration) are by weight, viscosity is kinematic viscosity and is measured at 100 °C in cSt, and temperatures are in degrees Celsius. Examples

Details of the gasoline compositions used in the following examples are given in Table 1 below. Table 1: Test Fuels

Test Fuel Composition

Fl Base fuel + 0.25 %vol high viscosity GtL base oil

F2 Base fuel + 0.50 %vol high viscosity GtL base oil

F3 Base fuel + 1.00 %vol high viscosity GtL base oil

F4 Base fuel 2.00 %vol high viscosity GtL base oil

In the examples, the base fuel used was an unleaded gasoline (95 ULG) of RON 95.1, MON 86.7, and having sulphur content (ASTM D 2622-94} of 8 ppmw, iso + normal paraffins content of 61.18 % v/v, saturated naphthenes content of 6.49%, aromatics content of 28.35% v/v and olefins content of 3.81 % v/v (as measured by Gas Chromatography), density at 15°C (ISO 3675: 93/D4052 : 91) 0.7307 kg/1, distillation (ISO 3405/88) IBP 39.2 ⁰C, 10% 56.6 ⁰C, 50% 93.2 ⁰C, 90% 139.8 ⁰C and FBP 172.2 ⁰C.

The High viscosity GtL base oil is a Fischer-Tropsch derived base oil having a kinematic viscosity at 100 ⁰C of 19.0 mm²/s (ASTM D 445), a viscosity index of approximately 170, a flashpoint of 247.5 ⁰C (ASTM D 93) and a pour point of -30 ⁰C (ASTM D 5950) .

The acceleration benefit of the fuels of Table 1 was measured using a four-stroke bench engine. The four- stroke bench engine used was a Ford Zetec 2.0 litre inline 4 cylinder, 16-valve, multipoint fuel injection, spark-ignition engine and an MT-75 gearbox, with electronic port fuel injection and mapped ignition system (EEC4) . For test bed operation, the Power Assisted Steering (PAS) pump was removed, and a different water pump (from a Ford Orion Zetec) fitted to accommodate the change in belt drive direction. The FM (Friction Modifier) acceleration test comprised a warm- up/stabilization (of dynamometer load, acceleration times, and engine/test cell temperatures) and then an acceleration test stage where a Base (reference) fuel is tested against a candidate fuel. The continuous test sequence is divided into distinct phases as follows:

"Base run #1", "Base run #2", "Test fuel run", "Base run #3".

Note. The acceleration cycles are run continuously throughout all the test phases, including fuel changes and flushing.

Engine oil, coolant and air intake temperatures, together with ambient conditions such as barometric air pressure and humidity are measured and monitored during the test, since these parameters can have an immediate and significant effect on acceleration test, which can be accounted for.

The details of the testing phase are as follows: The operator will conduct the test, starting the test using the base (reference) fuel and allow a minimum of: • 30 accelerations to be completed on warm- up/stabilization; and

• 45 accelerations to be completed on the base fuel (Base run#l) .

The operator will then carry out a simulated fuel change and perform a 5-minute flush procedure onto base fuel (base-to-base) , followed by:

• 45 further accelerations on Base fuel (Base run#2) . The operator will then carry out fuel change to the candidate (test) fuel, performing a 5-minute flush procedure onto candidate (test) fuel (base-to-test) , followed by:

. 45 accelerations on candidate (test) fuel (Test fuel run) . The operator will then carry out fuel change back to the base fuel, performing a 5-minute flush procedure back onto base fuel (test-to-base) , followed by: • 15 accelerations on base fuel (Base run#3) . The engine is then stopped and the test is ended.

To give statistical significance a single Test run consists of a minimum of 45 accelerations per fuel. Each candidate test fuel is tested in triplicate. The fuels were tested in a randomised order. During the test, the time taken for the engine to accelerate from 2010 to 4000 rpm is denoted as TmI, and the time taken for the engine to accelerate from 2300 rpm to 4000 rpm is denoted as Tm3. As detailed above, during the bench engine test, a large number of accelerations occur, enabling accurate determination of these two times. In addition, the engine is run with different fuels, in the order B-B-T-B (where B is the base fuel and T is the candidate (test) fuel) . This enables the acceleration benefit of two separate runs of the base fuel to be determined, which should be zero, and a difference from zero gives an indication of the level of accuracy of the test. To enable statistically significant results to be obtained, each test fuel was run through the test three times, and the test order of the different fuels was randomised. If TmI and Tπι3 are less for the test fuel compared to the base fuel, then the test fuel gives an acceleration benefit.

Tables 2 and 3 summarise the results from these tests, giving the value of TmI and Tm3 for each run of the test fuels, and also gives the average value of these parameters for each of the candidate (test) fuels. The results reported in Tables 2 and 3 give the acceleration benefit of the candidate (test) fuels of the present invention in terras of percentage improvement in acceleration performance over the base fuel.

Table 2: Improvement of TmI (%) Over Base Fuel

?able 3: Improvement of Tm3 (%) Over Base Fuel

It can clearly be seen from Tables 3 to 6 that an improvement in the acceleration performance of the test engine, as measured by TmI and Tm3_f is consistently- obtained for the test fuels in comparison with the base fuel.

Claims

_. o o _C L A I M S

1. A gasoline composition comprising:

(a) a major amount of gasoline; and

(b) from 0.1 to 5 wt%, based on the overall gasoline composition, of a base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt, which is derived from the product of a Fischer-Tropsch synthesis process.

2. A gasoline composition according to claim 1, wherein the amount of base oil is in the range of from 0.2 to 4.5 wt%, based on the overall gasoline composition. 3. A gasoline composition according to claim 2, wherein the amount of base oil is in the range of from 0.25 to

3.5 wt%, based on the overall gasoline composition.

4. A gasoline composition according to claim 3, wherein the amount of base oil is in the range of from 0.3 to 3.0 wt%, based on the overall gasoline composition.

5. A gasoline composition according to any one of claims 1 to 4, wherein the kinematic viscosity at 100 ⁰C of the base oil is in the range of from 8 to 60 cSt .

6. A gasoline composition according to claim 5, wherein the kinematic viscosity at 100 ⁰C of the base oil is in the range of from 12 to 40 cSt.

7. A gasoline composition according to any one of claims 1 to 6, wherein the gasoline composition additionally includes one or more non-Fischer-Tropsch derived base oil.

8. A gasoline composition according to any one of claims 1 to 1, wherein the gasoline composition additionally includes one or more fuel additive.

9. A process for preparing a gasoline composition, comprising bringing into admixture with gasoline, from 0.1 wt% to 5 wt%, based on the overall gasoline composition, of a base oil having a kinematic viscosity at 100 ⁰C of at least 1 cSt, which is derived from the product of a Fischer-Tropsch synthesis process.

10. A method of operating a four-stroke spark-ignition internal combustion engine, which method involves introducing into a combustion chamber of said engine a gasoline composition according to any one of Claims 1 to 8.