ZA200408311B

ZA200408311B - Diesel fuel compositions

Info

Publication number: ZA200408311B
Application number: ZA200408311A
Authority: ZA
Inventors: Richard H Clark; Christopher R Millington; Rodney Williams; Steven D Matchett
Original assignee: Shell Int Research
Priority date: 2002-04-25
Filing date: 2004-10-14
Publication date: 2006-07-26
Also published as: MY140444A; JP5179700B2; WO2003091364A2; DK1497398T3; AU2003229732B2; CN1656199A; AU2003229732A1; EP1497398B1; PL371868A1; CA2483200C; WO2003091364A3; NO20045114L; CN1656199B; US20050241216A1; AR043292A1; JP2005523973A; CA2483200A1; BR0309479A; EP1497398A2

Description

. .

Diesel fuel compositions : This invention relates to diesel fuel compositions, their preparation and their use in diesel engines, and to the use of certain types of fuel in diesel fuel compositions.

Typical diesel fuels comprise liquid hydrocarbon middle distillate fuel oils having boiling points from about 150 to 400°C. Examples of such fuels include the reaction products of Fischer-Tropsch methane condensation processes, for example the process known as Shell Middle

Distillate Synthesis (van der Burgt et al, “The Shell

Middle Distillate Synthesis Process”, paper delivered at the 5% Synfuels Worldwide Symposium, Washington DC,

November 1985; see also the November 1989 publication of the same title from Shell International Petroleum Company

Ltd, London, UK). These Fischer-Tropsch derived gas oils are low in undesirable fuel components such as sulphur, nitrogen and aromatics and tend to lead to lower vehicle emissions. They are typically blended with other diesel base fuels, for instance petroleum derived gas oils, at concentrations of for instance from 10 to 30% w/w, to modify the base fuel properties. ~ Compression-ignition (diesel) engines running on conventional diesel fuels can suffer from the build up of combustion related deposits in their fuel injection systems, in particular in the injector nozzles. This . injector fouling can impair engine performance. To reduce fouling, a detergent-containing additive may be included in

N WO 03/091364 PCT/EP03/04339 ~ . , the fuel, or the proportion of heavier components in the . | fuel may be adjusted so as to lower its endpoint.

It has now been found that Fischer-Tropsch derived : fuels can themselves contribute to a reduction in, and/or reversal of, injector fouling. A fuel composition containing such components can therefore be used to help maintain and/or improve engine cleanliness.

According to a first aspect of the present invention there is provided the use of a Fischer-Tropsch derived gas 0il in a diesel fuel composition, for the purpose of reducing subsequent combustion related deposits in a diesel engine into which the fuel composition is introduced.

The Fischer-Tropsch derived gas oil may instead or in addition be used for the purpose of removing previously incurred combustion related deposits (ie, to “clean up” the engine). .

In the context of the present invention, “reducing” includes complete prevention and “removing” embraces both complete and partial removal. “Use” of the Fischer-Tropsch derived gas oil in a fuel composition means incorporating the fuel into the composition, typically as a blend (ie, a physical mixture) with one or more other fuels, conveniently before the composition is introduced into the engine.

The Fischer-Tropsch derived gas oil should be suitable for use as a diesel fuel. Its components (or the majority, : for instance 95% w/w or greater, thereof) should therefore . have boiling points within the typical diesel fuel (“gas 0il”) range, ie, from about 150 to 400°C or from 170 to

~ , 370°C. It will suitably have a 90% w/w distillation ’ temperature of from 300 to 370°C.

By “Fischer-Tropsch derived” is meant that the fuel : is, or derives from, a synthesis product of a Fischer-

Tropsch condensation process. The Fischer-Tropsch reaction converts carbon monoxide and hydrogen into longer chain, usually paraffinic, hydrocarbons: n(CO + 2Hp) = (-CHp-)p + nH20 + heat, in the presence of an appropriate catalyst and typically at elevated temperatures (eg, 125 to 300°C, preferably 175 to 250°C) 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.

A gas oil product may be obtained directly from this reaction, or indirectly for instance by fractionation of a

Fischer-Tropsch synthesis product or from a hydrotreated

Fischer-Tropsch synthesis product. Hydrotreatment can involve hydrocracking to adjust the boiling range (see, eg,

GB-B-2077289 and EP-A-0147873) and/or hydroisomerisation . 20 which can improve cold flow properties by increasing the proportion of branched paraffins. EP-A-0583836 describes a two-step hydrotreatment process in which a Fischer-Tropsch synthesis product is firstly subjected to hydroconversion under conditions such that it undergoes substantially no © 25 isomerisation or hydrocracking (this hydrogenates the olefinic and oxygen-containing components), and then at . least part of the resultant product is hydroconverted under conditions such that hydrocracking and isomerisation occur : to yield a substantially paraffinic hydrocarbon fuel. The i - 4 - desired gas oil fraction (s) may subsequently be isolated ’ for instance by distillation.

Other post-synthesis treatments, such as ' polymerisation, alkylation, distillation, cracking- decarboxylation, isomerisation and hydroreforming, may be employed to modify the properties of Fischer-Tropsch condensation products, as described for instance in

US-A-4125566 and US-A-4478955.

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-0583836 (pages 3 and 4).

An example of a Fischer-Tropsch based process 1s the

SMDS (Shell Middle Distillate Synthesis) described in “The

Shell Middle Distillate Synthesis Process”, van der Burgt et al (supra). This process produces middle distillate range products by conversion of a natural gas (primarily methane) derived synthesis gas into a heavy long-chain oo hydrocarbon (paraffin) wax which can then be hydroconverted and fractionated to produce liquid transport fuels such as the gas oils useable in diesel fuel compositions. A version of the SMDS process, utilising a fixed-bed reactor for the catalytic conversion step, is currently in use in

Bintulu, Malaysia and its products have been used in petroleum derived gas oil blends in commercially available : automotive fuels.

Gas oils prepared by the SMDS process are commercially available for instance from the Royal Dutch/Shell Group of

. Ce

Companies. Further examples of Fischer-Tropsch derived gas . oils are described in EP-A-0583836, EP-A-1101813,

WO-A-97/14768, WO-A-97/14769, WO-A-00/20534, WO-A-00/20535, : WO-A-00/11116, WO-A-00/11117, WO-A-01/83406, WO-A-01/83641,

WO-A-01/83647, WO-A-01/83648 and US-A-6204426. i Suitably, in accordance with the present invention, the Fischer-Tropsch derived gas oil will consist of at least 70% w/w, preferably at least 80% w/w, more preferably at least 90% w/w, most preferably at least 95% w/w, of paraffinic components, preferably iso- and linear paraffins. The weight ratio of iso-paraffins to normal : paraffins will suitably be greater than 0.3 and may be up to 12; suitably it is from 2 to 6. The actual value for this ratio will be determined, in part, by the hydroconversion process used to prepare the gas oil from the Fischer-Tropsch synthesis product. Some cyclic paraffins may also be present. : . By virtue of the Fischer-Tropsch process, a Fischer-

Tropsch derived gas oil has essentially no, or undetectable levels of, sulphur and nitrogen. Compounds containing these hetercatoms 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 virtually no aromatic components.

The aromatics content of a Fischer-Tropsch gas oil, as determined by ASTM D 4629, will typically be below 1% w/w, preferably below 0.5% w/w and more preferably below 0.1% - w/w.

The Fischer-Tropsch derived gas oil used in the present invention will typically have a density from 0.76

) Z 6 - to 0.79 g/cm3 at 15°C; a cetane number (ASTM D613) greater ’ than 70, ‘suitably from 74 to 85; a kinematic viscosity from 2.0 to 4.5, preferably from 2.5 to 4.0, more preferably ’ from 2.9 to 3.7, centistokes at 40°C; and a sulphur content of 5 ppmw (parts per million by weight) or less.

Preferably it is a product prepared by a Fischer-

Tropsch methane condensation reaction using a hydrogen/carbon monoxide ratio of less than 2.5, preferably less than 1.75, more preferably from 0.4 to 1.5, and ideally using a cobalt containing catalyst. Suitably it will have been obtained from a hydrocracked Fischer-Tropsch synthesis product (for instance as described in

GB-B-2077289 and/or EP-A-0147873), or more preferably a product from a two-stage hydroconversion process such as that described in EP-A-0583836 (see above). In the latter case, preferred features of the hydroconversion process may be as disclosed at pages 4 to 6, and in the examples, of’

EP-A-0583836. :

The level of combustion related deposits in a diesel engine may be measured in its fuel injection system, with reference to the degree of fouling of the injector nozzles.

Degree of nozzle fouling may be assessed in a number of ways, for instance visually, by measuring the mass of deposits in a fouled nozzle or by measuring the fluid flow (for instance, fuel flow or more preferably air flow) ’ properties of the fouled nozzle relative to those of the clean nozzle.

An appropriate test might for example determine the degree of nozzle fouling (conveniently in the form of a percentage injector fouling index) under steady state

- 7 = conditions in a suitable diesel engine, for instance based ’ on the change in air flow rate through one or more of the nozzles as a result of using the fuel composition.

Conveniently the results are averaged over all of the injector nozzles of the engine. A suitable test protocol, using an indirect injection diesel engine, is described below in connection with the experimental examples. The : CEC standard test method F-23-T-00, which again involves injector nozzle air flow measurements, may also be used to assess engine fouling.

The invention may be 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.

The amount of the Fischer-Tropsch derived gas oil used may be from 0.5 to 100% w/w of the overall diesel fuel composition, suitably from 1 to 60% w/w, preferably from 5 to 50% w/w, more preferably from 10 to 30% w/w. It may be . desirable for the composition to contain 8% w/w or greater, more preferably 10% w/w or greater, most preferably 20% w/w or greater, of the Fischer-Tropsch gas oil.

Other fuel components of the composition may be diesel fuels of conventional type, typically comprising liquid hydrocarbon middle distillate fuel oils, for instance petroleum derived gas oils. Such fuel components will typically have boiling points within the usual diesel range of 150 to 400°C, depending on grade and use.

The diesel fuel composition may, in order to achieve a : reduction and/or removal of engine deposits, consist essentially of the Fischer-Tropsch derived gas oil - in other words it may contain a major proportion (by which is ] 5 meant preferably 99% w/w or more of the fuel composition, more preferably 99.5% w/w or more, most preferably 99.8% w/w or more, even up to 100% w/w) of the Fischer-

Tropsch derived gas oil, optionally with a minor proportion of one or more diesel fuel additives such as are known in the art but with no other diesel fuels.

The overall fuel composition 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, most preferably no more than 100 or 50 or even 10 ppmw. It preferably has a cetane number (ASTM D613) from 40 to 85, more preferably from 45 to 75. Its density will typically be from 0.75 to 0.9 g/cm3, preferably from 0.8 to 0.85 g/cm3, at 15°C.

The Fischer-Tropsch derived gas oil may in particular be used to enhance the performance of a fuel or fuel blend which would otherwise cause relatively high levels of combustion related deposits, for instance a fuel having a relatively high endpoint and/or containing relatively high levels of aromatic components, and/or of a fuel or blend which causes, after three hours’ engine running, a reduction in the achievable air flow rate through one or more of the engine nozzles of greater than 35 or 40 or 45% for instance measured using the test protocol described below.

- Cs

Although in general the diesel fuel composition may or may not contain additives, it is preferred that it includes a detergent, since this may enhance the cleaning effect of the Fischer-Tropsch derived gas oil. The first aspect of ] 5 the present invention may therefore involve the use of both a Fischer-Tropsch derived gas oil and a detergent, in a

E diesel fuel composition, for the purpose of reducing subsequent combustion related deposits in a diesel engine into which the fuel composition is introduced and/or for the purpose of removing previously incurred combustion related deposits.

By “detergent” is meant an agent (suitably a surfactant) which can act to remove, and/or to prevent the build up of, combustion related deposits within the engine,

Cs in particular in the fuel injection system such as in the injector nozzles. Such materials are sometimes referred to as dispersant additives.

The Fischer-Tropsch derived gas oil, or alternatively the combination of the Fischer-Tropsch derived gas oil and the detergent, is preferably included in the fuel composition at a concentration sufficient to achieve a reduction in engine fouling (measured for instance as outlined above) of at least 5%, preferably at least 8%, more preferably at least 10%, most preferably at least 20%, 25 . as compared to that which results from using (under the same or comparable conditions) the same fuel composition but without the Fischer-Tropsch gas oil. Alternatively the ’ reduction may be relative to the degree of engine fouling which results from the use (under the same or comparable conditions) of a fuel composition containing no, or less than 1% w/w, Fischer-Tropsch derived fuels and no, or less : than 50 or even 20 ppmw of, detergent. . More preferably, the Fischer-Tropsch derived gas oil, ) or alternatively the combination of the Fischer-Tropsch derived gas oil and the detergent, is included at a concentration sufficient to remove, at least partially, combustion related deposits which have built up in the engine's fuel injection system, in particular in the injector nozzles, during a previous period of running using another diesel fuel (typically a fuel containing no, or less than 1% w/w, Fischer-Tropsch derived fuels and no, or less than 50 or even 20 ppmw of, detergent), when the engine is subsequently run on the Fischer-Tropsch fuel containing composition. This concentration is preferably sufficient to remove at least 5% of the previously incurred injector deposits (measured for instance as described above), more preferably at least 10%, most preferably at least 15 or 20 or 25 or 30%. :

The removal of combustion related deposits may be achieved by running the engine on the Fischer-Tropsch fuel containing composition for instance for the same period of time as that during which the deposits accumulated, or more preferably for 75%, yet more preferably 50% or even 40% or 30%, of the period of deposit accumulation, conveniently under comparable conditions. Ideally at least partial removal of combustion related deposits is achieved by running the engine on the Fischer-Tropsch fuel containing : composition for five hours or less, preferably for three hours cor less, more preferably for two hours or less.

- Ca

Where the fuel composition includes a detergent, preferred concentrations lie in the range 20 to 500 ppmw active matter detergent based on the overall fuel ’ composition, more preferably 40 to 500 ppmw, most preferably 40 to 300 ppmw or 100 to 300 ppmw or 150 to 300 ppmw. In the case of most commercially available detergent containing diesel fuel additives, this may mean that the additive is incorporated at levels higher than the standard recommended single treat rate, for example between 1.2 and 3 times, preferably between 1.5 and 2.5 times, such as about twice the standard single treat rate. Lower detergent levels (for example, corresponding to between 0.5 and 1.2 times, preferably the same as, the standard single treat rate) may however be used to help reduce or prevent further engine fouling and/or power loss.

Examples of detergents suitable for use in accordance with the present invention include polyolefin substituted succinimides or succinamides of polyamines, for instance polyisobutylene succinimides or polyisobutylene amine succinamides, aliphatic amines, Mannich bases or amines and polyolefin (eg, 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-0557561 and WO-A-98/42808. Particularly preferred are polyolefin substituted succinimides such as polyisobutylene succinimides.

Detergent-containing diesel fuel additives are known and commercially available, for instance from Infineum (eg,

F7661 and F7685) and Octel (eg, OMA 4130D).

The fuel composition may contain other components in : addition to the Fischer-Tropsch derived fuel and if applicable the detergent. Typically such components will ) be incorporated in fuel additives, for instance in combination with a detergent. Examples are lubricity : enhancers such as EC 832 and PARADYNE™ (ex Infineum),

HITEC™ E580 (ex Ethyl Corporation) and VEKTRON™ 6010 (ex

Infineum) and amide-based additives such as those available from the Lubrizol Chemical Company, for instance LZ 539 C; dehazers, eg, alkoxylated phenol formaldehyde polymers such as those commercially available as NALCO™ EC5462A (formerly 7D07) (ex Nalco), and TOLAD™ 2683 (ex Petrolite); anti- foaming agents (eg, the polyether-modified polysiloxanes commercially available as TEGOPREN™ 5851 and Q 25907 (ex

Dow Corning), SAG™ TP-325 (ex 0Si) and RHODORSIL™ (ex Rhone - Poulenc)); ignition improvers (cetane improvers) (eg, 2- ethylhexyl nitrate (EHN), cyclohexyl nitrate, di-tert-butyl peroxide and those disclosed in US-4208190 at column 2, line 27 to column 3, line 21); anti-rust agents (eg, that ~~ sold commercially by Rhein Chemie, Mannheim, Germany as “RC 4801”, 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, eg, the pentaerythritol diester of polyisobutylene-substituted succinic acid); corrosion : inhibitors; reodorants; anti-wear additives; anti-oxidants (eg, phenolics such as 2,6-di-tert-butylphenol, or phenylenediamines such as N,N’'-di-sec-butyl-p- oo : phenylenediamine); and metal deactivators.

Unless otherwise stated, the (active matter) : concentration of each such additional component in the overall fuel composition is preferably up to 1% w/w, more preferably in the range from 5 to 1000 ppmw, advantageously from 75 to 300 ppmw, such as from 95 to 150 ppmw.

It is particularly preferred that a lubricity enhancer be included in the fuel composition, especially when it has a low (eg, 500 ppmw or less) sulphur content. The lubricity enhancer is conveniently present at a concentration from 50 to 1000 ppmw, preferably from 100 to 1000 ppmw, based on the overall fuel composition.

The (active matter) concentration of any dehazer in the fuel composition will preferably be in the range from 1 to 20 ppmw, more preferably from 1 to 15 ppmw, still more

CC preferably from 1 to 10 ppmw and advantageously from 1 to 5 ppmw. The (active matter) concentration of any ignition improver present will preferably be 600 ppmw or less, more . preferably 500 ppmw or less, conveniently from 300 to 500 ppmw. :

A second aspect of the present invention provides a method of operating a diesel engine, and/or a vehicle which is driven by a diesel engine, which method involves introducing into a combustion chamber of the engine a diesel fuel composition incorporating a Fischer-Tropsch derived gas oil, and optionally also a detergent, for the : purpose of reducing subsequent combustion related depecsits in the engine and/or removing previously incurred ’ 30 combustion related deposits in the engine.

"preferred features of the second aspect of the . invention, for instance regarding the engine type, the nature of the diesel fuel composition, the nature and : concentration of the Fischer-Tropsch derived gas oil and if present the detergent, as well as of other components of the fuel composition, and the ways in which engine fouling may be assessed, may all be as described above in connection with the first aspect.

According to a third aspect of the invention, there is . 10 provided a diesel fuel composition which includes a major proportion of a fuel or fuel blend for an internal combustion engine of the compression ignition type, wherein the fuel or fuel blend comprises at least 30% w/w of a

Fischer-Tropsch derived gas oil, preferably at least 40%

W/W, more preferably at least 50% w/w, most preferably at least 60% w/w. The fuel or. fuel blend may comprise up to 100% w/w of the Fischer-Tropsch derived gas oil, preferably up to 95% w/w, more preferably up to 90% w/w, most preferably up to 80% w/w or 70% w/w. :

This fuel composition preferably also contains a minor proportion of a detergent-containing additive. By “minor proportion” is meant preferably less than 1% w/w of the fuel composition, more preferably less than 0.5% w/w (5000 ppmw) and most preferably less than 0.2% w/w (2000 ppmw); references to “major proportion” may be construed accordingly.

As described above, in accordance with the present : invention a fuel or fuel blend may be additivated (additive-containing) or unadditivated (additive-free). If : 30 additivated, it will contain a minor proportion of one or id WO 03/091364 PCT/EP03/04359 more additives, in particular a detergent containing : additive. Such additives may be added at various stages during the production of the fuel composition; those added : at the refinery for example might be selected from anti- static agents, pipeline drag reducers, flow improvers (eg, ethylene/vinyl acetate copolymers or acrylate/maleic anhydride copolymers} and wax anti-settling agents (eg, those commercially available under the Trade Marks “PARAFLOW” (eg, PARAFLOW™ 450, ex Infineum), “OCTEL” (eg,

OCTEL™ W 5000, ex Octel) and “DODIFLOW” (eg, DODIFLOW™ wv 3958, ex Hoechst).

In accordance with a fourth aspect of the invention, there is provided a method of operating a diesel engine, and/or a vehicle which is driven by a diesel engine, which method involves introducing into a combustion chamber of the engine a diesel fuel composition according to the third aspect.

A fifth aspect of the invention provides a process for the preparation of a diesel fuel composition, such as a composition according to the third aspect, which process involves blending a Fischer-Tropsch derived gas oil with a non Fischer-Tropsch derived diesel fuel, optionally together with a detergent. Again the blending is ideally carried out with the aim of reducing subsequent combustion related deposits in a diesel engine into which the fuel composition is introduced and/or for the purpose of removing previously incurred combustion related deposits in : the engine.

The discovery that Fischer-Tropsch gas oils can at ) 30 least partly remove existing engine deposits may be put to use in the “clean-up” of a fouled engine. Thus, according : to a sixth aspect of the present invention, there is provided the use of a Fischer-Tropsch derived gas oil, ’ and/or of a fuel composition containing a Fischer-Tropsch derived gas o0il, to clean (ie, to remove combustion related deposits from) the fuel injection system of a diesel engine. “Use” in this way means running the engine, or a part thereof such as its fuel injection system, on the gas 0il or fuel composition for a period of time sufficient to effect at least partial removal of the combustion related " deposits. It need not necessarily involve driving the vehicle.

A Fischer-Tropsch derived gas oil, or a fuel composition containing such a gas oil, may therefore in accordance with the invention be packaged together with instructions for its use to clean a diesel engine in the manner described above.

The sixth aspect of the invention also encompasses a method of cleaning the fuel injection system of a diesel engine, by introducing into a combustion chamber of the engine a Fischer-Tropsch derived gas oil and/or a fuel composition containing a Fischer-Tropsch derived gas oil.

Preferred features of the third to the sixth aspects of the invention, for instance regarding the nature and concentrations of the Fischer-Tropsch derived gas oil, any detergent present and any other fuel components and additives present, may be as described above in connection : with the first and second aspects of the invention.

i -17 -

According to a seventh aspect, the present invention . provides a method for assessing the performance of a candidate diesel fuel composition, comprising the steps of: : 1) measuring the level of combustion related deposits in a diesel engine running on a “standard” diesel fuel composition, which “standard” fuel composition contains no, or less than 1% w/w of, Fischer-Tropsch derived gas oils; 2) subjecting the engine to a first test cycle running on the standard fuel composition; . 3) measuring the level of combustion related deposits in the engine after the first test cycle; 4) calculating the increase in deposits during the first test cycle; 5) subjecting the engine to a second test cycle running on the candidate diesel fuel composition; 6) measuring the level of combustion related deposits in the engine after the second test cycle; 7) calculating the increase in deposits (if any) during the second test cycle; and ©0020 8) if applicable, calculating the extent of removal of deposits during the second test cycle.

The “standard” fuel composition suitably contains no, or less than 50 or even 20 ppmw, active matter detergent.

It is suitably a low or ultra low sulphur diesel fuel, as described above, and is preferably unadditivated.

The level of combustion related deposits may be measured by assessing the degree of fouling of the injector . nozzles in the fuel injection system of the engine, as described above.

i WO 03/091364 PCT/EP03/04359

The test cycles involve running the engine on the ‘ relevant fuel composition for a given period of time and/or for a given number of miles. The tests may be conducted on the engine alone or driving a vehicle - in the latter case they may be conducted under simulated driving conditions (such as using a chassis dynamometer) or involve regular road driving (preferably under urban as opposed to motorway conditions). The engine running and/or driving conditions should be the same or comparable for both the first and the second test cycles. : By way of example, the duration of the first test cycle should be sufficient to cause a significant, and at least detectable, build up of combustion related deposits “compared to that measured in step 1 of the test. A typical first test cycle might last from 1 to 5 hours, preferably 2 hours or more, more preferably 3 hours or more.

An appropriate duration for the second test cycle is typically from 10 to 100%, preferably from 50 to 100%, most suitably 100%, of that of the first -test cycle. It may in cases be 80% or 75% or even 50% or less of the duration of the first test cycle. For assessing reductions in (as opposed to removal of) combustion related deposits, it may be up to 120% or 150% or even 200% of the duration of the first test cycle.

The engine used for the test may for instance be an indirect injection diesel engine, such as a Volkswagen™

Passat™ engine, for instance the Passat™ AAZ 1.9 TD engine. ’ The test may be conducted on only a part of the engine (eg, the fuel injection system) or on a simulated engine or engine part.

The assessment method of the invention is particularly applicable to a candidate diesel fuel composition which incorporates a Fischer-Tropsch derived gas oil, more : particularly to one which also incorporates a detergent.

The method may therefore be used to identify and/or evaluate fuel compositions according to the third aspect of the invention.

The method may also be used to assess the performance of a diesel engine, and/or to assess the performance of a fuel injection system for use in a diesel engine, and/or to assess the performance of a vehicle driven by a diesel : engine.

An eighth aspect of the present invention provides a diesel fuel composition which, when used as the candidate fuel composition in the assessment method of the seventh aspect, leads to removal of at least 5%, preferably at least 10% or 15% or 20% or 25% or 30%, of the combustion related deposits accumulated in the engine prior to step 5 of the test, when the duration of the second test cycle is the same as or less than, more preferably 80% or 75% ox even 50% or less of, the duration of the first test cycle, and the duration of the first test cycle is preferably at least 2 hours, more preferably 3 hours or more.

Such a fuel composition ideally contains, in accordance with the present invention, a Fischer-Tropsch derived gas oil, preferably together with a detergent.

The present invention will be further understood from ’ the following examples, which illustrate the effects of using Fischer-Tropsch derived gas oils in diesel fuel compositions, on the degree of fuel injector fouling.

Claims

1. Use of a Fischer-Tropsch derived gas oil in a diesel fuel composition, for the purpose of reducing subsequent combustion related deposits in a diesel engine into which the fuel composition is introduced and/or of removing previously incurred combustion related deposits from the engine.

2. Use according to claim 1, wherein the amount of the Fischer-Tropsch derived gas oil used in the fuel composition is 10% w/w or greater.

3. Use according to claim 1 or claim 2, wherein the fuel composition includes a detergent.

4. A method of operating a diesel engine, and/or a vehicle which is driven by a diesel engine, which method involves introducing into a combustion chamber of the engine a diesel fuel composition incorporating a Fischer- Tropsch derived gas oil, and optionally also a detergent, for the purpose of reducing subsequent combustion related deposits in the engine and/or removing previously incurred combustion related deposits in the engine.

5. Use of a Fischer-Tropsch derived gas oil, and/or of a fuel composition containing a Fischer-Tropsch derived gas oil, to remove combustion related deposits from a diesel engine.

6. Use according to claim 5, wherein the Fischer-Tropsch derived gas oil and/or the fuel composition is used together with a detergent.

7. A method for assessing the performance of a candidate . diesel fuel composition, comprising the steps of: 1) measuring the level of combustion related ’ deposits in a diesel engine running on a standard diesel fuel composition, which standard fuel composition contains no, or less than 1% w/w of, Fischer-Tropsch derived gas oils; 2) subjecting the engine to a first test cycle running on the standard fuel composition; 3) measuring the level of combustion related deposits in the engine after the first test cycle; 4) calculating the increase in deposits during the first test cycle; ) 5) subjecting the engine to a second test cycle running on the candidate diesel fuel composition; 0) measuring the level of combustion related deposits in the engine after the second test cycle; 7) calculating the increase in deposits (if any) during the second test cycle; and 8) if applicable, calculating the extent of removal of deposits during the second test cycle.

8. A diesel fuel composition which, when used as the candidate fuel composition in a method according to claim 7, leads to removal of at least 5% of the combustion related deposits accumulated in the engine prior to step 5 of the test, when the duration of the second test cycle is the same as or less than that of the first test cycle.

- 9. A diesel fuel composition which includes a major proportion of a fuel or fuel blend for an internal ’ 30 combustion engine of the compression ignition type, wherein the fuel or fuel blend comprises at least 30% w/w of a Fischer-Tropsch derived gas oil.

10. A diesel fuel composition according to claim 9, additionally comprising a detergent.

11. Use according to claim 1 or 5, substantially as herein described and exemplified and/or described with reference to the examples.

12. A method according to claim 4 or 7, substantially as herein described and exemplified and/or described with reference to the examples.

13. A diesel fuel composition according to claim 8 or 9, substantially as herein described and exemplified and/or described with reference to the examples. AMENDED SHEET