WO1997044415A1

WO1997044415A1 - Gasoline detergent compositions

Info

Publication number: WO1997044415A1
Application number: PCT/GB1997/001397
Authority: WO
Inventors: Trevor John Russell; Antony Cooney
Original assignee: The Associated Octel Company Limited
Priority date: 1996-05-23
Filing date: 1997-05-21
Publication date: 1997-11-27
Also published as: AU2909197A; GB9610781D0

Abstract

A gasoline comprising: (i) from 10 to 5000 ppm by weight of a polyisobutenyl succinimide wherein the polyisobutenyl substituent has a number average molecular weight (Mn) in the range 500 to 5000; (ii) from 10 to 5000 ppm by weight of a polyalkylene glycol of formula R'-O-(RO)n-R'' wherein R' and R'' independently represent hydrogen atoms or hydrocarbyl groups of up to 30 carbon atoms, having a molecular weight in the range 500 to 5000 and R represents a C2-8 alkylene radical; and (iii) from 10 to 5000 ppm by weight of a poly(C2-C6 olefin) wherein the polyolefin has a number average molecular weight (Mn) of 300 to 5000, and additive compositions for use in the preparation thereof.

Description

GASOLINE DETERGENT COMPOSITIONS

FIELD OF THE INVENTION

This invention relates to multi-functional detergent- containing additive compositions for hydrocarbon fuels. More particularly, the invention relates to alkenyl succinimide-based detergent compositions for hydrocarbon fuels and especially gasoline.

BACKGROUND AND DESCRIPTION OF PRIOR ART

It is well known in the art that during the operation of an internal combustion engine, fuel and lubricant deposits accumulate at various points in the inlet system. It is known that in order to avoid these disadvantages, multi-functional detergent-containing additive compositions are used which have to satisfy a large number of criteria, including:

(1) Elimination of carburettor and injector fouling.

(2) Good detergency in the inlet port and valve region.

An important property of fuel additive compositions, in addition to meeting the above criteria, is the avoidance of undesirable effects on engine operation such as prevention of valve stem sticking and tolerance of the package to water.

Valve stem sticking is a problem often associated with the use of high molecular weight detergents, particularly those with a polyisobutene backbone. The detergent layer on the valve stem may become so viscous under low temperature conditions that the valves stick. This leads to compression loss and even engine failure. Another potential problem is that of the package separating in the presence of water. In some circumstances additive storage conditions can be such that contamination with water accumulating at the bottom of storage tanks may result in the separation of package components.

Both of these problems are thought to be related to the miscibility between the detergent and carrier. Therefore it is particularly advantageous that compatibility between the additives is such that the combinations of detergent and carrier fluid do not lead to phase separation.

WO93/20170 discloses gasoline compositions containing a polyisobutenyl succinimide detergent wherein the polyisobutenyl substituent of the succinimide has a number average mol.wt. in the range 500-5000, and a carrier oil component comprising a mono end capped polypropylene glycol ether having a mol.wt. in the range 500-5000 or an ester thereof.

Certain combinations of polyisobutenyl succinimides and mono end capped polypropylene glycol ethers/esters are not compatible with each other. As those skilled in the art will appreciate it is possible to produce a broad range of fully miscible formulations by appropriate combination of particular succinimide detergents and particular polyoxyalkylene carrier fluids.

EP 588429 discloses a gasoline composition comprising a minor amount of a polyalphaolefin having a viscosity at 100°C of 2-20cst, and a minor amount of a polyoxyalkylene compound selected from glycols, mono and diethers thereof, having a number average mol.wt. in the range 400-3000. The composition additionally contains a minor amount of an ashless dispersant, preferably a polyolefin substituted succinimide wherein the polyolefin has a number average mol.wt. in the range 800-5000. The patent discloses that instability/immiscibility between a particular succinimide and a particular polyoxyalkylene compound can be overcome by the addition of the polyalphaolefin component.

US 3,676,089 discloses a liquid hydrocarbon fuel containing an alkenyl succinimide and a polymer or copolymer of a C₆-C₆ unsaturated hydrocarbon.

US 5,298,039 discloses a gasoline composition comprising

(a) from 10-5000ppm of a nitrogen containing detergent component which is or contains a polyisobutylamine and

(b) from 10-5000ppm of a dialkyl phenol initiated alkoxylate. The thrust of this patent is that full miscibility is obtained between the detergent and the dialkyl alkoxylate compared to the mono alkyl alkoxylate which is immiscible with the detergent.

US 5,405,419 discloses the use of polyolefin amine detergents, polyolefins and mono end capped polypropylene glycols in detergent compositions.

GB 2261441 discloses a fuel composition for controlling engine deposits which comprises (a) a polyoxyalkylene compound having a mol.wt. of 200-5000 and (b) a hydrocarbyl succinimide. An additive concentrate containing (a) and (b) may be made up with a mineral oil.

US 3,901,665 discloses a liquid hydrocarbon fuel composition comprising (a) from 0.01-0.06%wt of a 3 or 4 carbon olefin, preferably polyisobutene with a mol.wt. from 400-1400, preferably about 400-900 and (b) from 0.008-0.016%wt of a polyoxyalkylene compound of formula R(OCH2CH(CH3) )xOH, where R is alkyl, preferably B97 01 97

- 4 -

C₁₀_₁₈ and x has a value of 4-20. The additives are effective as anti-icing agents, as carburettor detergents and can reduce intake valve deposits.

US 3,658,494 discloses a fuel composition comprising an additive combination of (a) an oxy compound which is a monoether of a glycol or polyglycol and (b) a dispersant which could be a succinimide. In the examples the oxy compounds are typically ethylene glycol monobutyl ether.

JP 5117671 discloses an additive composition to reduce intake valve deposits comprising a mixture of mono and bis succinimides, a polyoxyalkylene glycol or an ether thereof and a lubricating oil fraction having a viscosity of 3-35cst at 100°C The polyoxyalkylene glycols and their ethers are of average mol.wt. of 500- 5000.

OBJECT OF THE INVENTION

It is an object of the present invention to provide combinations of additives for gasoline engines which are highly effective detergents for example synergistically effective in detergency, and are compatible with each other i.e. do not separate thereby ensuring no problems with valve stem sticking, package instability and water tolerance.

SUMMARY OF INVENTION

The present invention provides a novel fuel additive composition comprising a combination of a polyisobutenyl succinimide as a detergent/dispersant and a carrier fluid which is a combination of a polyalkylene glycol and a polyolefin, whereby the polyolefin may be partly replaced by a mineral oil.

Among other factors, the present invention is based on the surprising discovery that the unique combination of a polyisobutenyl succinimide, a polyalkylene glycol and a polyolefin provide compositions that are highly effective, for example synergistically effective in detergency, and also compatible with one another i.e. do not separate. Moreover, a poorly performing existing polyisobutenyl succinimide/polyether system can be improved by the addition of a polyolefin.

DETAILED DECRIPTION

The present invention thus provides a gasoline comprising (i) from 10 to 5000 ppm by weight, preferably 50 to 1000 ppm by weight, of a polyisobutenyl succinimide wherein the polyisobutenyl substituent has a number average molecular weight (Mn) in the range 500 to 5000, (ii) from 10 to 5000 ppm by weight, preferably 50 to 1500 ppm by weight, of a polyalkylene glycol of formula R' -0- (RO)n-R" where R* and R" independently represent hydrogen atoms or hydrocarbyl groups of up to 30 carbon atoms, having a molecular weight in the range 500 to 5000 and R represents a C₂._e alkylene radical, and (iii) from 10 to 5000 ppm by weight, preferably 50 to 1500 ppm by weight, of a poly(C₂-C₆ olefin) wherein the polyolefin has a number average molecular weight (Mn) of 300 to 5000, preferably 300 to 2000 and more preferably 300 to 1300, whereby the polyolefin may be partly replaced by a mineral oil.

The invention also provides an additive composition suitable for addition to a hydrocarbon fuel, particularly a gasoline, comprising (i) a polyisobutenyl succinimide detergent wherein the polyisobutenyl substituent of the succinimide has a number average molecular weight (Mn) in the range 500 to 5000; (ii) a polyalkylene glycol of formula R' -0- (RO)n-R" where R' and R" independently represent hydrogen atoms or hydrocarbyl groups of up to 30 carbon atoms, having a molecular weight in the range 500 to 5000 and R represents a C₂.₈ alkylene radical; (iii) a poly(C₂-C₆) - olefin as defined above and optionally a mineral oil; and (iv) optionally a hydrocarbon solvent having a boiling point in the range 66 to 270°C;

wherein the weight ratio of [ (ii) + (iii) ] to (i) is in the range 0.2:1 to 5:1, the weight ratio of (ii) to (iii) is in the range 0.1:1 to 10:1, and the hydrocarbon solvent is present in an amount from 0 to 70% by weight of the total composition, preferably 20 to 70% by weight.

As used herein, "gasoline" refers to motor fuels meeting ASTM Standard D-439, and includes blends of distillate hydrocarbon fuels with oxygenated fuels, such as ethanol, as well as the distillate fuels themselves. The fuels may be leaded or unleaded, and may contain, in addition to the additive compositions of this invention, any of the other additives conventionally added to gasolines as, for example, scavengers, anti-icing additives, octane requirement improvers etc.

DETERGENT

As indicated, the detergent component in this invention is a polyisobutenyl succinimide obtained by reacting a polyisobutenyl substituted succinic acid or anhydride with a polyalkylenepolyamine.

The polyisobutenyl substituent of the succinimide will generally have a number average molecular weight within the range 500-5000, preferably 800-1300.

The preparation of the polyisobutenyl substituted succinic anhydrides may be effected by reacting a polyolefin of appropriate molecular weight (with or without chlorine) with maleic anhydride by analogy to processes known in the art. Suitable processes include thermally reacting a polyolefin with maleic anhydride (see for example US patents 3,361,673 and 3,018,250) and reacting a halogenated, in particular a chlorinated, polyolefin with maleic anhydride (see for example US patent 3,172,892). Alternatively, the alkenyl substituted succinic anhydride can be prepared by mixing the polyolefin with maleic anhydride and passing chlorine through the mixture (see for example GB 949,981) .

The alkyl or alkenyl substituted succinimideε may be made by reaction of the desired hydrocarbyl substituted succinic acid or anhydride with a suitable polyalkylene polyamine of formula:

H₂N- (RNH)_n-R-NH₂

where R is an alkylene radical from 1 to 5 carbon atoms; n is an integer whose value or average value is 1 to 10, preferably 1 to 6. The preferred polyalkylene polyamines are polyethylene polyamines of formula:

H_?N(CH_?CH₂NH)_XH

where x is 1 to 6. They include for example ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine and pentaethylene hexamine. As will be appreciated this reactant may be a commercially available mixture which can contain amounts of branched and cyclic species.

Usually the polyisobutenyl substituted succinic acid or anhydride will be reacted with the polyalkylenepolyamine in a molar ratio from 0.2:1 to 5:1, preferably 0.2:1 to 2.5:1 and most preferably from 1:1 to 2:1. The reaction will usually be carried out at a temperature of at least 80°C, and preferably at a temperature in the range 125 to 250°C. Usually the detergent component will be added to the additive compositions of this invention in admixture with an aromatic solvent and containing from 20-70% by weight or more of the active detergent.

As used herein, the term succinimide is meant to encompass all the reaction products which may be obtained by reacting the hydrocarbyl substituted succinic acid or anhydride with the polyalkylenepolyamine such as amides, salt linkages etc, in addition to the imide linkage of the type that results from the reaction of the primary amino group and the anhydride moiety. It will be appreciated that commercially available materials may contain a greater or lesser amount of such alternative linkage reaction products and these commercially available mixtures are of course included in the term "succinimide".

CARRIER OIL

The polyoxyalkylene compounds may be represented by the following formula

R' -O- (RO)n-R"

where R' and R" independently represent hydrogen atoms or hydrocarbyl groups of up to 30 carbon atoms, i.e. including glycols, mono and diethers thereof, having a molecular weight in the range 500 to 5000. R represents a C₂__θ alkylene radical, preferably propylene.

The preferred compounds are polypropylene glycol monoethers represented by the formula,

R'0(CH₂CH(CH₃)0)_nH

where R' is a hydrocarbyl group of up to 30 carbon atoms, preferably straight chain C₁.₃₀ alkyl, more preferably straight chain C₄_₂₀ alkyl and most preferably C₁₂_₁₈ alkyl, and n is an integer whose value or average value is in the range 10-50, preferably 12 to 20.

Suitable monomers for the polyolefin are ethene, propene, butenes, isobutenes and pentenes. These monomers may form homopolymers or copoiymers. Polypropylenes and polyisobutenes are most preferred.

Polyisobutenes are especially preferred and can be obtained by polymerisation of a C„ fraction containing a high proportion of isobutene. Strictly speaking, since these materials contain small proportions of but-1-ene and but-2-ene the resulting polymers should be called simply polybutenes, but the term polyisobutenes is common in the art and will be used herein in this broad sense.

DILUENT

An additional component of the additive compositions of this invention may be a diluent or solvent added primarily to reduce the viscosity of the mix, thereby to improve its handling properties and to facilitate the blending of the additive with the gasoline. Generally the solvent will be an aromatic hydrocarbon having a boiling point in the range 66 to 270°C, e.g. toluene or xylene or more especially the aromatic solvent mixtures sold by Shell under the trade marks Shellsol AB, Shellsol R and by Esso under the trade mark Solvesso 150, and boiling in the range 180 to 270°C. The amount of solvent to be incorporated will depend upon the desired final viscosity, but will usually be from 20 to 70% of the final composition on a weight basis.

MINOR INGREDIENTS

As indicated, the gasolines of the present invention will usually contain a number of minor ingredients, P T/GB97/01397

- 10 - often added to meet specific customer requirements. Included amongst these are dehazers, usually an alkoxylated phenol formaldehyde resin, added to minimise water adsorption and to prevent a hazy or cloudy appearance, and a corrosion inhibitor, usually of the type comprising a blend of one or more fatty acids and amines. Either or both can also be present in the additive compositions of the present invention in amounts ranging from 1 to 5%, usually 1 to 3% each, based on the total weight of the composition.

Other specific purpose minor ingredients which may be added to the gasolines or to the additive compositions include anti-oxidants, anti-icing agents, metal deactivators, dyes and the like.

In general terms the total amount of such minor functional ingredients in the compositions will not exceed about 10% by weight, more usually not exceeding about 5% by weight.

Such minor additives are conventional in the art.

The additive compositions of this invention are blended into gasoline in amounts sufficient to provide from 10 to 5000 ppm (weight basis) of active detergent in the gasoline. Preferred amounts range from 50 to 1000 ppm. The quantity of each of the carrier oil components incorporated in the gasoline will usually be in the range 10 to 5000 ppm (weight basis) , preferably 50 to 1500 ppm.

The invention will be further understood from the following illustrative examples which are not to be construed in any way as limiting the scope of the invention. In the examples which follow, amounts of detergent are quoted as amounts of solution.

(1) Compositions containing a detergent, a polvalkylene glycol and a polyolefin

Additive concentrates in accordance with the invention and comparative examples were prepared and the results are presented in Table 1. PG is polyalkylene glycol. PIB is polyisobutene. The aromatic solvent used throughout is Shellsol AB as mentioned above.

EXAMPLE DETERGENT PG PIB COMMENTS

1 6.7 4 _ immiscible

2 6.7 2 2 clear

3 6.7 3 1 immiscible

4 6.7 1 3 clear

6 6.7 4 4 clear

6a 6.7 4 4 immiscible

7 10 20 - immiscible

8 10 10 10 immiscible

8a 10 5 15 clear

9 10 2 - clear

10 10 4 - immiscible

11 10 6 - immiscible

14 6.7 4 - immiscible

15 6.7 2 2 clear

16 8.3 5 _ clear

17 8.3 5 - clear

18 6.7 8 - turbid

19 6.7 7 1 turbid

20 6.7 6 2 clear

21 6.7 4 - immiscible

22 6.7 2 2 clear

27 6.7 4 - immiscible

28 6.7 2 2 clear

30 6.7 4 - immiscible

31 6.7 2 2 clear

42 6.7 4 _ clear

43 6.7 4 - clear

44 6.7 4 - immiscible

48 6.7 1 3 clear 49 6.7 4 4 clear

50 6.7 4 4 immiscible

51 10 10 10 clear

compnR-ition DescriDtions

Compositions 1-11 contain a detergent which is a 60% w/w solution in an aromatic solvent of the reaction product of a polyisobutene succinic anhydride in which the polyisobutene has an average mol.wt. from 940-1000 and a polyamine which is tetraethylene pentamine in a molar ratio of anhydride: amine of 1.7:1.0. The carrier is a polypropylene glycol monoether prepared using a C12-15 alcohol initiator, having a mol.wt. in the range 1200- 1500 and a kinematic viscosity at 40°C in the range 55-66 cst. The polyisobutene used has a number average mol.wt. of 440 and a viscosity at 100°C of 13 cst.

Composition 6a contains a polyisobutene of higher number average mol.wt., around 780, and a viscosity at 100°C of 107 cst.

Compositions 14 and 15 contain a 60% w/w solution in an aromatic solvent of polyisobutene succinimide derived from a polyisobutene succinic anhydride in which the polyisobutene has a number average mol.wt. from 940-1000 and a polyamine which is tetraethylene pentamine in a molar ratio of 2.0:1.0.

Composition 16 contains a 60% w/w solution in an aromatic solvent of polyisobutene succinimide derived from a polyisobutene succinic anhydride in which the polyisobutene has a number average mol. wt. around 780 and a polyamine which is tetraethylene pentamine in a molar ratio of 1.0:1.0.

Composition 17 contains a 60% w/w solution in an aromatic solvent of polyisobutene succinimide derived from a polyisobutene succinic anhydride in which the polyisobutene has a number average mol. wt. around 780 and a polyamine which is tetraethylene pentamine in a molar ratio of 2.0:1.0.

Compositions 18-20 contain a 60% w/w solution in an aromatic solvent of polyisobutene succinimide derived from a polyisobutene succinic anhydride in which the polyisobutene has a number average mol.wt. around 780 and a polyamine which is tetraethylene pentamine in a molar ratio of 1.6:1.0.

Compositions 21 and 22 contain a 60% w/w solution in an aromatic solvent of polyisobutene succinimide in which the polyisobutene has a number average mol.wt. of around 1300 and a polyamine which is tetraethylene pentamine in a mole ratio of 1.6:1.0.

Compositions 27 and 28 contain a 60% w/w solution in an aromatic solvent of polyisobutene succinimide in which the polyisobutene has a number average mol.wt. of 940- 1000 and a polyamine which is pentaethylene hexamine in a molar ratio of 1.6:1.0.

Compositions 30 and 31 contain a 60% w/w solution in an aromatic solvent of polyisobutene succinimide in which the polyisobutene has a number average mol.wt. of 940- 1000 and a polyamine which is diethylene triamine in a molar ratio of 1.6:10.

Composition 42 contains a 60% w/w solution of a polyisobutene succinimide derived from 940-1000 mol.wt. polyisobutene and tetraethylene pentamine in a molar ratio of 1.7:1.0. The carrier is a polypropylene glycol monoether prepred using a C12-15 alcohol initiator, having a mol.wt. around 700 and a kinematic viscosity at 40°C around 28 cst. Composition 43 contains the same detergent as in example 42 but the carrier is a polybutene glycol monoether prepared from a C12 alcohol initiator and has a kinematic viscosity at 40°C of 41 cst.

Compositions 44 and 48 contain the same detergent as in example 42 but the carrier fluid is a polypropylene glycol monoether having a viscosity at 40°C of 95 cst.

Compositions 49 and 51 contain a polyisobutene around 350 number average mol.wt. and a viscosity at 100°C of 4 cst.

Composition 50 contains a polyisobutene around 950-1000 number average mol.wt. and a viscosity of 203 cst at 100°C.

Water tolerance testing

Compositions are evaluated by shaking 20ml of the package under investigation with 1ml of water. After leaving overnight in a freezer at -18"C the packages are observed for signs of separation. Any separation is regarded as a fail by this test.

Composition Result

1 Fail

2 Pass

3 Fail 5 Pass

21 Fail

22 Pass

27 Fail

28 Pass 30 Fail 31 Pass

42 Pass

43 Pass (2) Compositions in which the polyolefin is part_ly replaced bv a mineral nil

Additive concentrates in accordance with the invention and comparative examples were prepared and the results are presented in Table 1. PG is polyalkylene glycol. PIB is polyisobutene. The aromatic solvent use throughout is Shellsol AB as mentioned above.

EXAMPLE DETERGENT PG PIB MIN.OIL COMMENTS

1 6.7 4 immiscible 2 6.7 2 1 1 clear 3 6.7 2 2 clear

4 6.7 4 4 immiscible 5 6.7 4 2 2 clear 6 10 20 immiscible

7 10 10 10 immiscible 8 10 10 5 5 clear 9 10 2 clear

10 10 4 "~ immiscible

14 6.7 4 immiscible 15 6.7 2 1 1 clear 16 6.7 4 2 2 clear 1 6.7 4 immiscible 2 6.7 2 1 1 clear 7 6.7 4 immiscible 8 6.7 2 1 1 clear 0 6.7 4 immiscible 1 6.7 2 1 1 clear 2 6.7 4 - clear 3 6.7 4 - clear 4 6.7 4 immiscible 5 6.7 2 1 1 clear 6 6.7 4 2 2 immiscible 7 6.7 2 1 1 clear 8 6.7 4 2 2 clear 9 6.7 4 2 2 clear Composition Descriptions

Compositions 1-10 contain a detergent which is a 60% w/w solution in an aromatic solvent of a polyisobutene succinic anhydride in which the polyisobutene has an average mol.wt. from 940-1000 and a polyamine which is tetraethylene pentamine in a molar ratio of anhydride: amine of 1.7:1.0. The carrier is a polypropylene glycol monoether prepared using a C12-15 alcohol initiator, having a mol.wt. in the range 1200-1500 and a kinematic viscosity at 40°C in the range 55-66 cst. The polyisobutene used has a number average mol.wt. of 440 and a viscosity at 100°C of 13 cst. The mineral oil is a "HVI" base oil having a viscosity at 100°C of around 4.5 cst and a viscosity index of 101.

Compositions 4 and 5 contain a polyisobutene of number mol.wt. around 780 and a viscosity at 100°C of 107 cst.

Compositions 14, 15 and 16 contain a 60% w/w solution in an aromatic solvent of polyisobutene succinimide derived from a polyisobutene succinic anhydride in which the polyisobutene has a number average mol.wt. from 940-1000 and a polyamine which is tetraethylene pentamine in a molar ratio of 2.0:1.0.

Composition 21 and 22 contain a 60% w/w solution in an aromatic solvent of polyisobutene succinimide in which the polyisobutene has a number average mol. wt. of around 1300 and a polyamine which is tetraethylene pentamine in a molar ratio of 1.6:1.0.

Compositions 27 and 28 contain a 60% w/w solution in an aromatic solvent of polyisobutene succinimide in which the polyisobutene has a number average mol. wt. of 940- 1000 and a polyamine which is tetraethylene pentamine in a molar ratio of 1.6:1.0. Compositions 30 and 31 contain a 60% w/w solution in an aromatic solvent of polyisobutene succinimide in which the polyisobutene has a number average mol.wt. of 940- 1000 and a polyamine which is diethylene triamine in a molar ratio of 1.6:1.0.

Composition 42 contains a 60% w/w solution of a polyisobutene succinimide derived from 940-1000 mol.wt. polyisobutene and tetraethylene pentamine in a molar ratio of 1.7:1.0. The carrier is a polypropylene glycol monoether prepared using a C12-15 alcohol initiator, having a mol.wt. around 700 and a kinematic viscosity at 40°C around 28 cst.

Composition 43 contains the same detergent as in example 42 but the carrier is a polybutene glycol monoether prepared from a C12 alcohol initiator and has a kinematic viscosity at 40°C of 41 cst.

Compositions 44-46 contain a carrier fluid which is a polypropylene glycol monoether having a viscosity at 40°C of 95 cst.

Compositions 47 and 48 contain a polyisobutene around 350 number average mol.wt. and a viscosity at 100°C of 4 cst.

Composition 49 contains a mineral oil having a viscosity of 4 cst at 40°C.

Water tolerance testing

Compositions are evaluated by shaking 20ml of the package under investigation with 1ml of water. After leaving overnight in a freezer at -18°C the packages are observed for signs of separation. Any separation is regarded as a fail by this test. Composition Result

1 Fail

2 Pass

15 Pass

16 Pass 44 Fail 47 Pass

Claims

1. A gasoline comprising:

(i) from 10 to 5000 ppm by weight of a polyisobutenyl succinimide wherein the polyisobutenyl substituent has a number average molecular weight (Mn) in the range 500 to 5000;

(ii) from 10 to 5000 ppm by weight of a polyalkylene glycol of formula R' -0- (RO)n-R" wherein R¹ and R" independently represent hydrogen atoms or hydrocarbyl groups of up to 30 carbon atoms, having a molecular weight in the range 500 to 5000 and R represents a C_2-8 alkylene radical; and

(iii) from 10 to 5000 ppm by weight of a poly(C₂-C₆ olefin) wherein the polyolefin has a number average molecular weight (Mn) of 300 to 5000.

2. A gasoline according to claim 1 comprising from 50 to 1000 ppm by weight of the polyisobutenyl succinimide.

3. A gasoline according to claim 1 comprising from 50 to 1500 ppm by weight of the polyalkylene glycol.

4. A gasoline according to claim 1 comprising from 50 to 1500 ppm by weight of the poly(C₂-C₆ olefin) .

5. A gasoline according to claim 4 wherein said polyolefin has a number average molecular weight (Mn) in the range 300 to 2000.

6. A gasoline according to claim 5 wherein said number average molecular weight (Mn) is in the range 300 to 1300.

7. A gasoline according to any of the preceding claims wherein the polyolefin may be partly replaced by a mineral oil.

8. An additive composition suitable for addition to a hydrocarbon fuel comprising:

(i) a polyisobutenyl succinimide detergent wherein the polyisobutenyl substituent of the succinimide has a number average molecular weight (Mn) in the range 500 to 5000;

(ii) a polyalkylene glycol of formula R¹ -0- (RO)n-R" wherein R' and R" independently represent hydrogen atoms or hydrocarbyl groups of up to 30 carbon atoms, having a molecular weight in the range 500 to 5000 and R represents a C₂__B alkylene radical; and

(iii) a poly(C₆-C₆ olefin) wherein the polyolefin has a number average molecular weight (Mn) in the range 300 to 5000.

9. An additive composition according to claim 8 wherein said polyolefin has a number average molecular weight (Mn) in the range 300 to 2000.

10. An additive composition according to claim 9 wherein said number average molecular weight (Mn) is in the range 300 to 1300.

11. An additive composition according to claim 8 further comprising a hydrocarbon solvent having a boiling point in the range 66 to 270°C.

12. An additive composition according to claim 11 wherein the weight ratio of [ (ii) + (iii)] to (i) is in the range 0.2:1 to 5:1, the weight ratio of (ii) to (iii) is in the range 0.1: to 10:1, and the hydrocarbon solvent is present in an amount from 0 to 70% by weight of the total composition.

13. An additive composition according to claim 12 wherein said hydrocarbon solvent is present in an amount from 20 to 70% by weight of the total composition.

14. An additive composition according to any one of claims 8 to 13 wherein the polyisobutenyl substituent of the succinimide has a number average molecular weight (Mn) in the range 800 to 1300.

15. An additive composition according to claim 8 wherein the polyalkylene glycol is represented by the formula:

R'-O- (CH₂CH(CH,)0)_nH

wherein R' is a hydrocarbyl group of up to 30 carbon atoms and n is an integer whose value or average value is in the range 10 to 50.

16. A composition as claimed in any one of the preceding claims further comprising a number of minor functional ingredients in an amount not exceeding 10% by weight.