WO2002022766A1

WO2002022766A1 - Fuel composition

Info

Publication number: WO2002022766A1
Application number: PCT/GB2001/003762
Authority: WO
Inventors: Alisdair Quentin Clark
Original assignee: Bp Oil International Limited
Priority date: 2000-09-15
Filing date: 2001-08-21
Publication date: 2002-03-21
Also published as: GB0022709D0; AU2001282301A1

Abstract

Unleaded aviation fuel compositions having a Motor Octane Number of at least 98, for use in piston driven aircraft comprising triptane and/or tertiary butyl benzene and at least one other saturated liquid aliphatic hydrocarbon having from 5 to 10 carbon atoms. The compositions preferably contain triptane, iso-pentane and either one or any combination of; iso-octane, toluene and methyl tertiary butyl ether and tert butyl benzene.

Description

FUEL COMPOSITION

The present invention relates to fuel compositions and in particular aviation gasolines possessing a high octane number suitable for use in piston-driven aircraft.

If a gasoline engine is run on a fuel which has an octane number lower than the minimum requirement for the engine, knocking will occur. Straight run gasoline has a low motor octane number but may be boosted to the required motor octane number of 82-88 for automotive use by the addition of octane boosters such as tetraethyl lead either alone or with refinery components such as reformate, alkylate, cracked spirit or chemical streams such as toluene, xylene, methyl tertiary butyl ether or ethanol. Aircraft piston-driven engines operate under extreme conditions to deliver the desired power e.g. high compression ratios. Due to the severity of the conditions e.g. with turbo charging or super charging the engine, aviation piston-driven engines require fuel of a minimum octane level higher than that for automotive internal combustion gasoline engines, in particular at least 98-100. The base fuel of an aviation gasoline has a motor octane number of 90-93. To boost the motor octane number sufficiently to the required level, tetraethyl lead is added to the aviation base fuel. The fuel may contain the organolead and also other octane boosters, such as those described above. Industrial and Engineering Chemistry Vol. 36 No. 12 pl079-1084 dated 1944 describes the use of triptane (2,2,3-trimethylbutane) in combination with tetraethyl lead in aviation gasoline. However, the presence of tetraethyl lead is the key to achieving high octane quality in aviation gasolines.

In modern day formulations tetraethyl lead is always used to boost the octane quality of the aviation gasoline to the desired level. However due to environmental concerns of the effect of lead and its compounds attempts are being made to find an alternative to the use of tetraethyl lead in aviation gasoline. Conventional octane boosters such as ethers, aromatics, such as toluene, and non-lead metal compounds can boost the motor octane number of unleaded motor gasoline sufficiently high enough to achieve the desired value but they do not boost the motor octane number of an unleaded aviation gasoline sufficiently high enough to ensure satisfactory performance or suffer from other significant technical limitations.

US 5470358 describes the use of aromatic amines to boost the motor octane number of unleaded aviation gasoline to at least 98 but many aromatic amines are known to be toxic. They have high boiling points, high freezing points and; they are also prone to produce gums.

There remains a need for an unleaded aviation gasoline of sufficiently high octane number suitable for use in piston driven aircraft

According to the present invention there is provided an unleaded aviation fuel composition, having a Motor Octane Number of at least 98, and usually a final Boiling

Point of less than 200°C or especially 170°C, and preferably a Reid Vapour Pressure at

37.8°C of between 35-60 especially 38-60 kPascals, which comprises: component (a) at least one hydrocarbon having the following formula I R-CH2-CH(CH₃)-C(CH3)₂-CH3 I wherein R is hydrogen or methyl and component (b) at least one saturated liquid aliphatic hydrocarbon having 4 to 10 in particular 5 or 6 carbon atoms optionally with at least one other saturated liquid aliphatic hydrocarbon having from 5 to 10 carbon atoms wherein 35-92% especially 40- 78% by volume of the total composition is a hydrocarbon of formula I. Unless otherwise stated all percentages in this specification are by volume, and disclosures of a number of ranges of amounts in the composition or gasoline for 2 or more ingredients includes disclosures of all sub-combinations of all the ranges with the ingredients.

IfR is hydrogen the hydrocarbon is triptane. IfR is methyl the hydrocarbon is 2,2,3 trimethylpentane. Especially preferred is triptane. Triptane and 2,2,3 trimethylpentane maybe used individually or in combination with each other, for example, in a weight ratio of 10:90 - 90:10, preferably, 30:70 - 70:30. The composition may comprise apart from a component (I), the hydrocarbon of formula I, a component (H) which is at least one of the known octane boosters described above especially an oxygenate octane booster, usually an ether, usually of Motor Octane Number of at least 96-105 e.g. 98-103. The ether octane booster is usually a dialkyl ether, in particular an asymmetric one, preferably wherein each alkyl has 1-6 carbons, in particular one alkyl being a branched chain alkyl of 3-6 carbons in particular a tertiary alkyl especially of 4-6 carbons such as tert-butyl or tert-amyl, and with the other alkyl being of 1-6 e.g. 1-3 carbons, especially linear, such as methyl or ethyl. Examples of component (π) include methyl tertiary butyl ether, ethyl tertiary butyl ether and methyl tertiary amyl ether. Cyclic ethers such as furan, tetra hydro furan and their lower alkyl e.g. methyl derivatives may also be used. The oxygenate may also be an alcohol of 1 -6 carbons e.g. ethanol.

At least one component (I) may be present together with at least one component (H) in a combination. The combination may be, for example, triptane together with methyl tertiary butyl ether. The combination may be in a volume ratio of 40:60 to 99: 1 e.g. 50:50 to 90 : 10, preferably 60:40 to 85:15. The volume percentage of ether may be up to 30% of the total composition e.g. 1-30%, such as 1-15% or 5-25%. The unleaded blend composition may also be substantially free of any oxygenate octane booster e.g. ether or alcohol. The motor octane number of the aviation gasoline of the invention is at least 98, for example 98-103, preferably 99 to 102 or especially 100-101.5. Motor Octane Numbers are determined according to ASTM D 2700-92. The hydrocarbons of formula I may also, especially when present in amount of at least 30% by volume, be used to provide gasolines of the invention with a Performance Number (according to ASTM D909) of at least 130 e.g. 130-170.

Triptane or 2,2,3 trimethylpentane may be used in a purity of at least 95% but is preferably used as part of a hydrocarbon mixture obtained, via distillation of a cracked residue, which is an atmospheric or vacuum residue from crude oil distillation, to give a C4 fraction containing olefin and hydrocarbon, alkylation to produce a C4.9 especially a C6_9 fraction which is distilled to give a predominantly Cg fraction, which usually contains trimethyl pentanes including 223 trimethyl pentane and/or 233 trimethyl pentane. To produce triptane this fraction can be demethylated to give a crude product comprising at least 5% of triptane, which can be distilled to increase the triptane content in the mixture; such a distillate may comprise at least 10% or 20% of triptane and 2,2,3 trimethylpentane but especially at least 50% e.g. 50-90% the rest being predominantly of other aliphatic C7 and C8 hydrocarbons e.g. in amount 10-50% by volume. Triptane may be prepared generally as described in Rec. Trav. Chim. 1939, Vol.

58 pp 347-348 by J P Wibaut et al, which involves reaction of pinacolone with methyl magnesium iodide followed by dehydration (e.g. with sulphuric acid) to form triptene, which is hydrogenated e.g. by catalytic hydrogenation to triptane. Alternatively triptane and 2,2,3 trimethylpentane may be used in any commercially available form. The invention will be further described with triptane exemplifying the compound of formula I but 2,2,3 trimethylpentane may be used instead or as well.

The amount of the hydrocarbon of Formula I alone or with component II may be present in the composition in an effective amount to boost the Motor Octane Number to at least 98 and maybe in a percentage of from 35-92%, preferably 60-90%, especially 70-90% by volume, based on the total volume of the composition. In particular the compound of formula I is usually in the composition in a percentage of 30-60% more especially 30-50% by volume, but based on the total composition, preferred are 40-90% or 50-90% or most especially 45-70%.

The composition also comprises a component (b). Component (b) is at least one saturated aliphatic liquid hydrocarbon of 4 to 10 preferably 5 to 8 in particular 5 or 6 carbon atoms, alone or with at least one saturated aliphatic liquid hydrocarbon (different from component(a)) having from 4 to 10 carbons in particular 5 to 10 carbon atoms, preferably 5 to 8 carbon atoms, especially in combination with one of 4 carbons. Component (b) may comprise a component (IH) which is more volatile and has a lower boiling point than component (a) in particular one boiling at least 30°C such as 30- 60°C below that of triptane at atmospheric pressure, and especially is itself of Motor Octane Number greater than 88 in particular at least 90 e.g. 88-93 or 90-92. Examples of component (in) include alkanes of 5 carbons in particular iso-pentane, which maybe substantially pure or a crude hydrocarbon fraction from alkylate or isomerate (eg of Bp 25-80°C) containing at least 30% e.g. 30-80% such as 50-70%, the main contaminant being up to 40% mono methyl pentanes and up to 50% dimethyl butanes. The amount of isopentane in the composition is usually 3-35% eg 5-35, 5-25, 5-15, 10-18% or 1- 10% such as 3-10%. When the isopentane is added to make the composition in the form of the crude fraction from alkylate or isomerate with at least 30% isopentane, the volume amount of alkylate fraction or isomerate may be 6-70%, eg 7-50% especially 6- 44, eg 6-17 or 10-44%. Component (HI) of boiling point 30-60°C less than that of triptane may be used as sole component (HI) but may be mixed with an alkane of boiling point 60-100°C less than that of triptane e.g. n and/or iso butane in blends of 99.5:0.5 to 50:50 such as 88:12 to 70:30, e.g. 88:12 to 75:25 or 70:30 to 50:50. Iso-pentane alone or mixed with n-butane is preferred, especially in the above proportions. In particular a volume amount of butane in the composition is up to 7% such as up to 6.5 or 5.5% e.g. up to 3.5% e.g. 1-3.5% or 2-3.5%, or 1.5-5.5% or 2-7 such as 3.5-5.5%

Component (b) may also comprise a component (IV) having a boiling point higher than component (a) preferably one boiling at least 18°C more than the compound of formula I e.g. triptane such as 20-60°C more than triptane but less than 170°C and usually is of Motor Octane Number of at least 92 e.g. 92-100; such components (IV) are usually alkanes of 7-10 carbons especially 7 or 8 carbons, and in particular have at least one branch in their alkyl chain, in particular 1-3 branches, and preferably on an internal carbon atom and especially contain at least one -C(CH₃)₂- group. An example of component (IV) is iso-octane.

The amount of component TV in particular isooctane (224 trimethylpentane) in the composition may be zero, but is usually 10-80% eg 12-48%, 10-35, 10-25, 35-60 or 45-75% but may be 1-25% e.g. 1-10% or 5-20%. The component IV especially isooctane may be added as such to form the composition, and/or may be added in the form of a fraction comprising at least 30% of said component IV especially isooctane such as 30-80% such as 40-60%; examples of such as fractions are alkylate fractions eg bp (1 bar pressure) of 85-135°C and 90-115°C or 95-105°C. Such fractions may be mixtures predominantly of branched chain eg iso C₈ hydrocarbons (eg at least 50% or 60% of the mixture) especially mixtures predominantly of iso C₇ and iso C₈ hydrocarbons and usually with small amounts (eg 1-20% (of the mixture) of either or both) of iso C₆ and iso C₉ hydrocarbons. Amounts of such fractions in the composition maybe 2-55% e.g. be 8-55% e.g. 12-52% or 2-15 or 5-15%. Blends of such fractions with added component IV eg isooctane may be used, in particular with 10-35% TV (eg isooctane) and 5-55% fractions eg alkylate fractions (especially predominantly iso C₈ hydrocarbon) such as 8-25%.

Component (b) may be a combination of at least one component (HI) together with at least one component (TV). The combination may be, for example, butane or isopentane together with iso-octane, and the combination may be in a volume ratio of 10 : 90 to 90 : 10, preferably 10:50 to 50:90, especially 15:85 to 35:65 or 15-50:85-50, in particular with butane or especially isopentane together with iso-octane. Especially preferred is the combination of isopentane together with iso-octane, in particular, in the above proportions, and optionally butane.

In another preferred embodiment, triptane and isopentane and optionally n- butane are present in the composition of the invention with 80-90% triptane and in particular in relative volume ratios of 80-90:10-15:0-3.5.

In a preferred embodiment of this invention component (a) is 2,2,3 trimethylbutane and component (b) is isopentane in combination with iso-octane, preferably in relative volume ratios of 10-80 : 5-25 : 10-80 in particular 30-50 : 5- 25 : 35-60 or 15-45 : 10-18 : 45-75 or 60-80 : 10-18 : 10-25. Especially the composition contains 30-80% of triptane and the isopentane and iso-octane are in a volume ratio of 35-15 : 65-85.

In a most preferred embodiment the composition of the invention comprises as Component (a) 223 trimethyl butane in an amount of 40-90% and as component (b) an isomerate fraction comprising 30-70% isopentane (the amount of isomerate being 6- 47% in the composition, isooctane in amount of 10-35% and 1-3.5% butane, the isooctane being present as such and/or mixed with other hydrocarbons in an isooctane containing fraction. Especially preferred compositions comprise 40-60% triptane, 6- 17% isomerate, 10-35% isooctane, 1-3.5% butane, the isooctane being especially at least partly (eg at least 20% such as 30-60%) present in a mixture predominantly of iso C₇ and iso C₈ hydrocarbons, with small amounts of iso C₆ and iso C₉ hydrocarbons (said mixture providing 8-55% of the total volume of the composition).

In a further preferred embodiment of this invention the composition comprises component (a) as 2,2,3 trimethylbutane, methyl tertiary butyl ether and component (b) as isopentane in combination with n-butane, preferably in relative volume ratios of 50- 90 : 5-30 : 10-15 : 0.1-3.5 in particular 50-80 : 10-25 : 10-15 : 0.1-3.5.

If desired the composition may comprise an aromatic liquid hydrocarbon of 6-9 e.g. 6-8 or 7-9 carbons, such as xylene or a trimethyl benzene, preferably toluene, in particular in amounts of up to 30% by volume of the total composition e.g. 1-30% or 5- 30%, such as 5-20% or 5-15% or 1-15% such as 2-15% e.g. 2-10%. In this case a preferred embodiment is a composition that may thus contain 15-95% or 15-90%, 50- 95% e.g. 15-80% or 50-80% triptane, 5-25% e.g. 10-25% component (b) e.g. isopentane and 5-30%, for example toluene. The benzene content of the composition is preferably less than 0.1% by volume. The gasoline composition suitable for avgas may also be substantially free of aromatic compound. Amounts of aromatic compounds of less than 42% or 40%, e.g. less than 35% or especially less than 30% or 20% are preferred. Preferably the amount of benzene is less than 5% preferably less than 1.5% or 1% e.g. 0.1-1% of the total volume or less than 0.1% of the total weight of the composition. The aromatic hydrocarbon(s) is preferably in an reformate fraction e.g. of bp 100-140°C.

In another preferred embodiment the composition may comprise both the aromatic hydrocarbon and the ether or just the aromatic hydrocarbon. In this case a preferred composition may comprise 45-80% triptane 0% or 5-30% ether (with a preferred total of both of 70-85%), and either with 10-25% component (b) (HI) e.g. isopentane (optionally containing butane) and 5-20% toluene, all by volume, or with 3-15% component (b) HI of the total of isopentane and butane (if present) and 2-15% toluene and 1-20% such as 5-15% tert-butyl benzene. The compositions may also comprise 10-90% e.g. 25-85%, 35-80%, or 35-90% by volume of triptane, 5-75% e.g. 8-55% by volume of a mixture predominantly of iso Cη and iso Cg hydrocarbons, but usually with small amounts of iso Cg and iso C9 hydrocarbons and 5-40% e.g. 8-40% or 5-35% or 8-25% by volume isopentane. The triptane and mixture may be obtained as a distillation fraction obtained by the processing of crude oil and subsequent reactions as described above.

Other compositions of the invention comprise by volume (i) 60-90% e.g. 70-85% triptane, (ii) 2-20% of component HI or an alkane of 4-7 carbons (or mixture thereof), at least the majority of which boils below triptane, such as 2-10% isomerate or 5-20% isopentane, (iii) 0 or up to 15% such as 2-15% liquid aromatic hydrocarbon e.g. toluene or xylene or a mixture of hydrocarbons containing at least a majority thereof, e.g. substantially all aromatics as in a reformate fraction (e.g. of boiling point 105-135°C) and (iv) 0 or up to 15% e.g. 2-15% of component IV which may be isooctane or an alkylate fraction (e.g. of bp 95-105°C), and (v) 0 or up to 7% e.g. 2-7% butane.

Composition suitable for use in formulated avgas may comprise the compound of formula 1 e.g. triptane with as component (b) at least one of isomerate and alkylate especially a cut boiling at 90-170°C e.g. 95-125°C, especially both, and in particular in volume ratios of 1 :4 to 4: 1 e.g. 1:1 to 1:3. Examples of such compositions contain (and preferably consist essentially of 40-80% such as 50-70% triptane and 20-60% of said component (b), in particular both isomerate and the alkylate, especially with at least 5% of each e.g. 5-40% such as 5-20% (e.g. of isomerate) and 15-35% (e.g. of alkylate cut). The compositions of the invention may also contain an aromatic compound containing a benzene nucleus substituted by at least 1 (e.g. 1 or 2 especially 1) branched chain alkyl substituent of 3-5 carbon atoms i.e. a secondary or especially tertiary alkyl group hereinafter called component I¹ . More than 1 group may be present of the same or a different type and in o, m or p position. Examples of such groups are isopropyl, isobutyl, secbutyl, tertbutyl, isoamyl, sec amyl, neopentyl and tertamyl; tertiary butyl is preferred, so the preferred compound is tert butyl benzene. The volume amount of this subsituted aromatic compound maybe 0% or 1-30% such as 2-25 e.g. 5-15%.

Examples of unleaded aviation gasoline compositions with such or substituted aromatic compound are ones with 2-7% e.g. 3.5-5.5% butane 0% or 1-15 such as 3-10% isopentane, 50-90% triptane especially 50-70% or 70-90%, 0% or 1-25% e.g. 1-10 or 5- 20% or 10-25% isooctane, 0%, 1-15% or 2-15% e.g. 2-10% toluene, 0% or 5-30% asymmetric dialkylether such as methyl tert butyl ether or especially ethyl tert butyl ether, and 1-20% eg. 5-15% tert butyl benzene. Such compositions can have Reid Vapour Pressuer at 37.8°C of 35-50kPa, while MON is usually 99.5-104 e.g. 100-102. Such branched chain alkyl substituted benzenes are commercial available materials and and may be made by known means. Thus they may be made by alkylation of benzene with an olefin of 3-5 carbons especially one with a branch methyl or ethyl group or an internal olefinic carbon atom e.g. a 2-alkyl substituted olefin e.g. 2-methyl butene 1 (isobutene) or 2 ethyl butene-1 (iso pentene) or propylene. The alkylation is usually in the presence of a Friedel Crafts or Bronsted Acid catalyst e.g. iron or aluminium chloride or sulphuric acid or boron trifluoride. The alkylation gives predominantly monosubstitution especially with the tert butyl group, but there may be some e.g. up to 10% di-substituted product e.g. in o or p position; the crude alkylation product may be used in the gasolines as such or after purification to 95% + purity.

The unleaded aviation gasoline composition of the invention usually has a net calorific value (also called Specific Energy) of at least 42MJ/kg (18075 BTU/lb) e.g. at least 43.5MJ/kg (18720 BTU/lb) such as 42-46 or 43.5-45MJ/kg. The gasoline usually has a boiling range (ASTM D86) of 25-170°C and is usually such that at 75°C, 8-40% such as 10-40% or 8-25% by volume is evaporated, at 105°C a minimum of 50% is evaporated e.g. 50-100 especially 85-100%, at 135°C a minimum of 90% e.g. 90-100% such as 96-100% is evaporated; the final boiling point is usually not more than 170°C preferably 80-140° or 80-130°C. The gasoline usually has a maximum freezing point of -40°C, in particular -55 or -60°C e.g. a freezing point of -40° to -90°C such as -70 to - 90°C. The Reid Vapour Pressure of the gasoline at 37.8°C measured according to ASTM D323 is usually 30-60kPa preferably 35-60 e.g. 38-55 or especially 38-49 or 45- 55kPa.

Unleaded gasoline compositions of the invention comprising a branched chain alkyl substituted benzene as described above usually have a boiling range (ASTM D86) of 30-200°C e.g. 35-190°C with an initial boiling point of 35-45°C, and are usually such that the temperature for distillation of 10% of the gasoline is 60-100°C e.g. 65-80°C or 80-90°C the 40% distillation temperature is at least 0.5 - 8°C greater e.g. 8-15°C greater, e.g. 75-110 such as 80-90 or 90-105°C, the 50% distillation temperature is usually at least 0.5°C higher e.g. 0.5-3°C higher such as 80-110 such as 81-91 or 95-105°C the

90% distillation temperature is at least 20°C higher still e.g. 20-120°C or 20-45°C or 40- 90°C higher, such as 105-190°C e.g. 105-130°C or 130-190°C such as 105-120°C or 115-130°C, the sum of the 10% and 50% distillation temperatures are usually 150-200, such as 150-165 or 180-195°C and the final boiling point of at least 50-75°C such as 50- 65°C higher than the 90% distillation figure, such as 175-195 e.g. 178- 190°C. The freezing point and RVP are usually as described above. These values for the gasolines with the substituted alkyl benzene usually apply whether the gasoline also contains compound (a) e.g. triptane or not.

The composition of the invention may contain at least one aviation gasoline additive, for example as listed in ASTM D-910 or DEF-STAN 91 -90; examples of additives are anti-oxidants, corrosion inhibitors, anti-icing additives e.g. glycol ethers or alcohols and anti-static additives, especially antioxidants such as one or more hindered phenols; in particular the additives maybe present in the composition in amounts of 0.1- lOOppm e.g. l-20ppm, usually of an antioxidant especially one or more hindered phenols. A coloured dye may also be present to differentiate the aviation gasoline from other grades of fuel. Aromatic amines e.g. liquid ones such as aniline or alkyl ones e.g. m-tohiidine may be present, if at all, in amount of less than 5% by volume, and are preferably substantially absent in the avgas compositions e.g. less than lOOppm. The relative volume ratio of the amine to triptane is usually less than 3:1 e.g. less than 1:2. The compositions of the invention may also contain other engine performance enhancing fluids, such as methanol/water mixtures (though these are preferably absent) or maybe used with nitrous oxide injection in the combustion air or cylinder.

The invention can provide aviation gasoline in particular of 99-102 MON values, with desired high Octane Levels but low emission values on combustion in particular of at least one of total hydrocarbons, total air toxics, NOx, carbon monoxide, and carbon dioxide, especially of both total hydrocarbons and NOx. Thus the invention also provides the use of a compound of formula I, in particular triptane, in unleaded aviation gasoline of MON of at least 98, e.g. as an additive to or component therein, to reduce the emission levels on combustion, especially of at least one of total hydrocarbons, total air toxics NOx, carbon monoxide and carbon dioxide especially both of total hydrocarbons and NOx. The invention also provides a method of reducing emissions of exhaust gases in the combustion of unleaded aviation gasoline of MON of at least 98 which comprises having a compound of formula I present in the fuel which is a gasoline of the invention. The invention also provides use of an unleaded gasoline of the invention in a spark ignition combustion engine to reduce emissions of exhaust gases. The invention also provides a method of reducing the exhaust gas temperature of a spark ignition combustion engine (e.g. an aviation engine) which comprises having a compound of formula 1 in the fuel which is combusted. The invention also provides the use of said compound to reduce the exhaust gas temperature of said engine in particular an air cooled aviation engine. In the compositions, gasolines, methods and uses of the invention the hydrocarbon of formula 1 , in particular triptane is preferably used in a emission-reducing effective amount, and/or in a exhaust - gas - temperature - reducing effective amount. While the compositions of the invention may be used in supercharged or turbocharged engines, they are preferably not so used, but are used in normally aspirated ones. The compound of formula I e.g. triptane may reduce one or more of the above emission levels better than amounts of alkylate or a mixture of aromatics and oxygenate at similar Octane Number and usually decrease the fuel consumption as well. The compositions and gasolines of the invention are unleaded and can have reduced toxicity compared to ones with aromatic amines or organo leads. In addition, contamination of the engine oil by toxic materials (e.g. lead compounds) is reduced and the fuel can be formulated to be highly immiscible with ground water.

As described above, the compound component I e.g. triptane or 2,2,3-trimethyl pentane may be used with the branched chain alkyl substituted benzene component I¹. The ratio of component I to I¹ being 0:1 to 100: 1 , such as 0: 1 or 1 :10 to 20: 1 especially 5-10: 1. Thus in a modification, component I¹ may be used in the substantial absence of compound I. The present invention also provides an unleaded aviation fuel composition having a MON value of at least 98, such as 99-102 and usually a final boiling point of less than 200°C e.g. 180-190°C and preferably an RVP at 37.8°C of between 38-60 kPa, which comprises component a ^l which is component I¹ and component (b) as defined above, wherein 1-30% of the composition by volume is said component I¹. The present invention also provides a formulated unleaded aviation gasoline, which comprises at least one aviation gasoline additive and said aviation fuel composition. In addition the present invention also provides the use of the compound component I¹ in unleaded aviation gasoline of MON at least 98 as an additive to or component therein to boost octane number of said gasoline. The present invention also provides a method of boosting octane number of an unleaded aviation gasoline, which comprising having said component I present in said gasoline. The composition and formulated gasoline containing component I¹ may contain the component H, HI, TV and/or an aromatic liquid hydrocarbon of 6-9 carbons, each substantially as described above.

The volume percentage of the component I¹ is usually 1-30% e.g. 5-28% such as 8-18 or 12-28%. The volume percentage of the ether maybe to 30% of the total composition e.g. 1-30% such as 1-15% or 5-25%. The unleaded composition may also be substantially free of any oxygenate octane booster e.g. the ether or an alcohol. The MON level of this modified gasoline is at least 98 e.g. 98-103, 99-102 or especially 101 and the Performance Number (measured according to ASTM D909) at least for those gasolines with 15-30% component I¹ of at least 130 e.g. 130-170. Component (b) may comprise component HI which has a boiling point less than 80°C, e.g. 30-60°C below at atmospheric pressure e.g. one described above, preferably an alkane of 5 carbons e.g. isopentane, which is usually present in the composition in 0% or 1-15 such as 3-10%. This component HI may be present with or substituted by an alkane of boiling point -20° to 20°C e.g. n or isobutane in blends of 0:1 to 10:1, such as 1:3 to 3:1 or 1 :2 to 2:1. The volume amount of the butane(s) in the composition is usually 1.5-10% e.g. 4-9%. The volume amount of component IV, preferably isooctane, is usually 35-80%, 45-75% such as 45-62% or 62-75%; the isooctane is preferably used substantially pure, rather than in a crude refinery fraction e.g. alkylate. Preferred blends contain butane(s): isopentane: isooctane in the volume ratios of 4-9:0-8:45-80, while preferred blends of these with tert butyl benzene are in the volume ratios 4-9:0-8:45-80:5-30. Blends of butane(s), isooctane and tert butylbenzene contain these in the volume ratio 4-9:55-75:15-30, and these blends may contain 10-20% of the ether component H.

The volume percentage of the aromatic liquid hydrocarbon (different from the branched chain component I¹) is usually 5-40% e.g. 8-35% such as 8-17% or 17-30%, with amounts of benzene less than 5% or 1% e.g. less than 0.1%. The total of the percentage of said liquid hydrocarbon and component I¹ is usually 10-35% e.g. 17-27%. Preferred compositions and gasolines of the invention with component I¹ but without component I comprise 1.5-10% of n and/or iso butane e.g. 4-9%, 0% or 1-15% such as 3-10% component HI e.g. isopentane, 35-80% e.g. 35-60 or 45-75% component IN e.g. isooctane, 5-40% e.g. 8-35 or 8-20% of one or more aromatic liquid hydrocarbons e.g. toluene and/or xylene (especially with less than 1% of benzene), 0 or 1-25% such as 5-25% of one or more asymmetric dialkylethers such as MTBE and ETBE and 1-25% such as 5-15% of component I¹ especially tertbutyl benzene. The pure aromatic hydrocarbon e.g. toluene or xylene may be replaced by a refinery fraction containing it e.g. a reformate fraction.

The physical properties of the unleaded gasolines with component I¹ are usually within the same ranges as those given above for gasolines with component I and I¹. The unleaded gasolines with component I¹ may be converted into unleaded formulated gasolines of the invention by addition of the aviation gasoline additive as described above in the described amounts.

The gasolines of the invention may be used in internal combustion spark ignition engines. They may be used to power moving vehicles in the air; the invention also provides a method of moving such vehicles by combustion of a gasoline of the invention. The vehicle usually has a driver and especially means to carry at least one passenger and/or freight.

The engines using aviation gasoline are usually in piston driven aircraft, i.e. with at least one engine driving a means for mechanically moving air such as at least one propeller. Each engine usually drives at least one propeller driving shaft with 1 or 2 propellors. The aircraft may have 1-10 propellers e.g. 2-4. The aircraft engines usually have at least 2 cylinders, e.g. 2 to 28 cylinders, each of which is preferably greater than 700cc in volume, such as 700-2000cc e.g. 1310cc. The total engine size is usually 3700-50000cc e.g. 3700 to 12000cc for single or twin engined passenger light aircraft, 12000 to 45000cc for 2 or 4 engined freight or airline use (e.g. 15-200 passengers, such as 50 to 150 passengers). The engines may have an engine power to weight ratio of at least 0.3Hp/lb wt of engine, e.g. 0.3-2Hp/lb, and may have a power to cylinder volume of at least 0.5 (Hp/cu.in) e.g. 0.5-2. Cylinders maybe arranged in rows, V formation, H formation, flat ('horizontally opposed') or radially around a common propeller drive shaft. One or more rows/circles of cylinders may be used, e.g. flat 2, flat 4, flat 6, VI 2, 1, 2 or 3 circles of 7 cylinders etc. Every cylinder has one and more preferably at least two spark plugs. A gear system may optionally be used to drive the propeller and or a supercharger. Alternatively, an exhaust turbo charger may also be present. Exhaust outlets may be individual or run into a common manifold and preferably point in the opposite direction to forward flight. Fins may be present on the exterior of the engine for air cooling. Greater than 90% of the distance travelled by the engine, when in use, is usually spent at 500 feet or more above ground level. Typically, during greater than 90% of the time when the engine is running, the engine operates at above lOOOrpm e.g. between 1000 to 3500 rpm. The invention may be used in conjunction with a fuelling system to control at least one of the cylinder head and exhaust gas temperatures during operation by adjustment of the air: fuel ratio, e.g. reducing this reduces the temperature. The aircraft usually has at least one tank having a capacity of at least 1001, especially with a total capacity of at least 10001. Small and micro-light aircraft may have tanks substantially smaller in capacity but can operate on the unleaded aviation gasoline described.

The gasolines of the invention may be made in a refinery or chemicals utility by blending the ingredients to produce at least 200,0001/day of gasoline such as l-10million 1/day. For aviation use, the gasoline is usually made in a refinery to produce at least 1000 barrels per day (or 100,0001/day) such as 0.1-2 million 1/day. The avgas is usually distributed by tanker by road, rail or water, or pipelines directly to the airport distribution or holding tanks, e.g. of at least 300,0001 capacity, from whence it is distributed by pipeline or tanker (e.g. a mobile refuelling bowser to fuel a plurality of aircraft, e.g. at least 5/day per tank; the aircraft may have one or more on-board tank each of at least 1001 capacity.

The invention will be illustrated by way of the following Examples. Example 1

An unleaded aviation gasoline blend was made by mixing 55% by volume of 223 trimethyl butane of 99% purity with 10% by volume of isomerate, (containing

54.8% isopentane 14.1% 2,2 dimethylbutane, 19.1% of 2 and 3 methylpentanes and the remainder other hydrocarbons of 5-10 carbons), 3% of volume of butane, 20% of isooctane(224 trimethyl pentane) and 12% of an alkylate fraction (bp 90-135°C containing 51 % isooctane, 21% other trimethyl pentanes and 22% mixed isomeric hydrocarbons.

The MON of the gasoline was 99.3 as determined by ASTMD 2700-92, the Reid Vapour Pressure was 40.9 kPa, the Supercharge Performance Number greater than 133 (determined from the Indicated Mean Effective Pressure EViEP/reference fuels - see ASTM D909), and the freezing point less than -80°C. A formulated unleaded aviation gasoline contained the above gasoline blend and

15 mg/1 of a phenol antioxidant 55% minimum 2,4 dimethyl-6-tertiary butyl phenol 15% minimum 4 methyl-2, 6-ditertiary-butyl phenol with the remainder as a mixture of monomethyl and dimethyl-tertiary butyl phenols (DEF STAN 91-90 RDE/A/610). The gasoline analysis is given in Table 1. The gasoline was also tested for carbon dioxide, carbon monoxide. No_x and total hydrocarbon emissions against a standard leaded aviation gasoline in a research engine operating at 42 rps/20.5 Nm and Lambda 1.15 (representing aircraft cruise conditions) with the ignition setting optimised for the standard gasoline. The emissions were reduced, the changes being -4.1% CO₂, -1.1% CO, -3.9% CO_x, -8.7% NO_x, -6.2% THC. The exhaust gas temperatures were an average of 617°C for the standard leaded fuel and 609°C for the gasoline of the invention.

Table 1

Examples 2-16

Unleaded aviation gasoline blends 1-15 were made by mixing the ingredients shown in Table 2. A corresponding series of formulated unleaded aviation gasolines contained the individual blends and lOmg/1 of the phenol antioxidant used in Example 1. The gasolines are tested for emissions on combustion and give reduced emissions compared to the standard leaded gasoline as in Ex.l. In the Table cut alkylate is an alkylate fraction boiling at 95-105°C containing a majority of isooctane and also 7-10 carbon alkanes, cut reformate is a reformate fraction boiling at 105-135°C and consisting of aromatics, in particular toluene and xylene and isomerate contains a majority of isopentane and also other 4-10 carbon alkanes. The physical properties of the cut alkylate cut refoπnate and isomerate are given in Table 1.

Table 1

Table 2

Examples 17-27

Unleaded aviation gasoline blends 1-11 were made by mixing the ingredients shown in Tables 3 and 4 and had properties as shown in the Tables; all were essentially free of benzene (<0.1% w/w). A corresponding series of formulated unleaded aviation gasolines containing the blends and lOmg/l of the phenol antioxidant of Ex.l were made. The gasolines are tested for emissions on combustion and give reduced emissions compared to the standard leaded avgas used in Example 1.

Table 3

Table 4

Claims

Claims:

1. An unleaded aviation fuel composition, having a Motor Octane Number of at least 98, which comprises: component (a) comprising at least one hydrocarbon having the following formula I R-CH2-CH(CH₃)-C(CH₃)2-CH₃ (I) wherein R is hydrogen or methyl and component

(b) at least one saturated liquid aliphatic hydrocarbon having 4-10 carbon atoms wherein at least 30% by volume of the total composition is a hydrocarbon of formula I.

2. A composition according to Claim 1 wherein the hydrocarbon of formula I is triptane.

3. A composition according to Claim 1 having a Reid Vapour Pressure at 37.8°C of 38 - 60 kPascals, preferably 38-49 kPascals.

4. A composition according to Claim 1 wherein component (b) comprises a component (Ifl) which is more volatile and has a lower boiling point than triptane, with a Motor Octane Number greater than 88, preferably iso-pentane.

5. A composition according to Claim 1 wherein the fuel composition comprises 5- 40% by volume of iso-pentane, preferably 5-25% by volume of iso-pentane.

6. A composition according to claim 4 wherein component (HI) comprises butane and iso-pentane.

7. A composition according to Claim 1 wherein component (b) comprises in addition to component (HI) a component (IV) having a boiling point higher than triptane but less than 170°C, with a Motor Octane Number of at least 92, preferably iso-octane.

8. A composition according to Claim 1 wherein the composition comprises up to 30% by volume of an aromatic liquid hydrocarbon of 6-8 carbons, preferably 5-30% by volume of toluene.

9. A composition according to claim 1 which comprises as component (I) said hydrocarbon having the formula I and a component (H) an oxygenate octane booster, which is an ether, preferably methyl tertiary butyl ether or ethyl tertiary butyl ether.

10. A composition according to Claim 7 which comprises 30-80% of triptane, and also HI isopentane and IN iso-octane, the volume ratio of HI to IV being 35-15 : 65-85.

11. A composition according to claim 6 which comprises 40-90% of triptane, an isomerate fraction comprising 30-70% isopentane the amount of isomerate being 6-44% of the composition, and in the composition also isooctane in amount of 10-35% of the composition and 1-3.5% of butane.

12. A composition according to claim 11 which comprises 40-60% triptane, 6-17% isomerate, 10-35% isooctane, 1- 3.5% butane, the isooctane being at least partly present in a mixture predominantly of iso C₇ and iso C₈ hydrocarbons with small amount of iso C₆ and iso C hydrocarbons, said mixture providing 8-55% of the total volume of the composition.

13. A composition according to Claim 8 which comprises 50-90% triptane, 5-25% isopentane and 5-30% toluene.

14. A composition according to claim 1 which comprises 1-30% by volume of a branched chain alkyl substituted benzene wherein a branched chain alkyl group is a secondary or tertiary alkyl group of 3-5 carbons, and there are 1 or 2 of such groups as substituents.

15. A composition according to claim 14 wherein said substituted benzene is tertbutyl benzene.

16. A composition according to claim 14 which comprises 2-7% butane, 1-15% isopentane, 50-90% triptane, 1-25% isooctane, 1-15% toluene, 5-25% of methyl tertbutyl ether or ethyl tert butyl ether, and 1-20% tertbutyl benzene.

17. An unleaded aviation fuel having a Motor Octane Number of at least 98, and a final boiling point less than 170°C which comprises: component (a) comprising at least one hydrocarbon having the following formula I R-CH2-CH(CH₃)-C(CH₃)2-CH₃ (I) wherein R is hydrogen or methyl and component

(b) at least one saturated liquid aliphatic hydrocarbon having 4 to 10 carbon atoms wherein at least 20% by volume of the total composition is a hydrocarbon of formula I, together with at least one aviation gasoline additive selected from anti-oxidants, corrosion inhibitors, anti-icing additives and anti-static additives.

18. An unleaded aviation fuel composition having a Motor Octane Number of at least 98 which comprises 1-30% (by volume) of component (h) a branched chain alkyl substituted benzene, having 1 or 2 branched chain secondary tertiary alkyl groups of 3-5 carbons, and (b) at least one saturated liquid aliphatic hydrocarbon having 4-10 carbon atoms.

19. A composition according to claim 18 which comprises 4-10% of butane 0 or 2- 10% isopentane, 45-75% isooctane, 0 or 8-35% toluene and 0% or 5-25% methyl or ethyl tert butyl ether, and 5-30% tertbutyl benzene.

20. An unleaded aviation fuel having a Motor Octane Number of at least 98 which comprises a fuel composition according to claim 18 and at least one aviation gasoline additive selected from antioxidants, corrosion inhibitors, anti-icing additives and antistatic additives.

21. A method of boosting the octane number of an unleaded aviation gasoline which comprises having present in said gasoline a compound of formula I or a branched chain alkyl substituted benzene having 1 or 2 branched chain secondary or tertiary alkyl groups of 3-5 carbons.

22. A method of reducing the exhaust gas temperature from combustion in a spark ignition aviation combustion engine of an unleaded aviation gasoline, which comprises having present in said gasoline a compound of formula I or a branched chain alkyl substituted benzene having 1 or 2 branched chain secondary or tertiary alkyl groups of 3-5 carbons.