WO2022180094A1

WO2022180094A1 - High octane unleaded aviation gasoline

Info

Publication number: WO2022180094A1
Application number: PCT/EP2022/054526
Authority: WO
Inventors: Hanane Belmokaddem Bennis; Timothy Michael Shea
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2021-02-24
Filing date: 2022-02-23
Publication date: 2022-09-01
Also published as: EP4298189A1; US20240076568A1; AU2022226388A1; CA3210705A1

Abstract

An unleaded aviation fuel composition having a MON of at least 99.6, sulfur content of less than 0.05 wt%,CHN content of at least 97.2 wt%, less than 2.8 wt% of oxygen content, a T10 of at most 75ºC, T40 of at least 75º C, a T50 of at most 105º C, a T90 of at most 135ºC, a final boiling point of less than 190°C, an adjusted heat of combustion of at least 43.5 MJ/kg, a vapor pressure in the range of 38 to 49 kPa, comprising from 20 vol.% to 35 vol.% of toluene having a MON of at least 107; from 2 vol.% to 10 vol.% of aniline; from above 30 vol% to 55 vol% of at least one alkylate oralkyate blend having an initial boiling range of from 32°C to 60°C and a final boiling range of from 105°C to140°C, having T40 of less than 99°C, T50 of less than 100°C, T90 of less than 110°C, the alkylate or alkylate blend comprising isoparaffins from 4 to 9 carbon atoms, 3-20 vol% of C5 isoparaffins, 3-15 vol% of C7 isoparaffins, and 60-90 vol% of C8 isoparaffins, based on the alkylate or alkylate blend, and less than 1 vol% of C10+, based on the alkylate or alkylate blend; at least 8 vol% of isopentane in an amount sufficient to reach a vapor pressure in the range of 38 to 49 kPa; from 0.1 vol% to 10 vol%, preferably from 1 vol% to 8 vol%, of a straight chain alkyl acetate having a straight chain alkyl group having 4 to 8 carbon atoms; and from 0.1 vol% to 10 vol%, preferably from 2 vol% to 8 vol%, of a branched chain alcohol having from 4 to 8 carbon atoms, provided that the branched chain does not contain any t-butyl groups; wherein the volume ratio of straight chain alkyl actetate to branched chain alcohol is in the range of 3:1 to 1:3; and wherein the fuel composition contains less than 15 vol% of C8 aromatics. As well as meeting the requirements of the ASTM D910 specification, the unleaded aviation fuel compositions ofthe present invention have improved octane properties.

Description

HIGH OCTANE UNLEADED AVIATION GASOLINE

Field of the Invention

The present invention relates to unleaded aviation gasoline fuel, in particular to unleaded aviation gasoline fuel having improved octane properties. Background of the Invention

Avgas (aviation gasoline), is an aviation fuel used in spark-ignited internal-combustion engines to propel aircraft. Avgas is distinguished from mogas (motor gasoline), which is the everyday gasoline used in cars and some non-commercial light aircraft. Unlike mogas, which has been formulated since the 1970s to allow the use of 3-way catalytic converters for pollution reduction, avgas contains tetraethyl lead (TEL), a non- biodegradable toxic substance used to prevent engine knocking (detonation).

Aviation gasoline fuels currently contain the additive tetraethyl lead (TEL), in amounts up to 0.53 mL/L or 0.56 g/L which is the limit allowed by the most widely used aviation gasoline specification 100 Low Lead (100LL). The lead is required to meet the high octane demands of aviation piston engines: the 100LL specification ASTM D910 demands a minimum motor octane number (MON) of 99.6, in contrast to the EN 228 specification for European motor gasoline which stipulates a minimum MON of 85 or United States motor gasoline which require unleaded fuel minimum octane rating (R+M)/2 of 87.

Aviation fuel is a product which has been developed with care and subjected to strict regulations for aeronautical application. Thus aviation fuels must satisfy precise physico-chemical characteristics, defined by international specifications such as ASTM D910 specified by Federal Aviation Administration (FAA). Automotive gasoline is not a fully viable replacement for avgas in many aircraft, because many high-performance and/or turbocharged airplane engines require 100 octane fuel (MON of 99.6) and modifications are necessary in order to use lower-octane fuel. Automotive gasoline can vaporize in fuel lines causing a vapor lock (a bubble in the line) or fuel pump cavitation, starving the engine of fuel. Vapor lock typically occurs in fuel systems where a mechanically-driven fuel pump mounted on the engine draws fuel from a tank mounted lower than the pump. The reduced pressure in the line can cause the more volatile components in automotive gasoline to flash into vapor, forming bubbles in the fuel line and interrupting fuel flow.

The ASTM D910 specification does not include all gasoline satisfactory for reciprocating aviation engines, but rather, defines the following specific types of aviation gasoline for civil use: Grade 80; Grade 91;

Grade 100; and Grade 100LL. Grade 100 and Grade 100LL are considered High Octane Aviation Gasoline to meet the requirement of modern demanding aviation engines. In addition to MON, the D910 specification for Avgas has the following requirements: density; distillation (initial and final boiling points, fuel evaporated, evaporated temperatures T10, T40, T90, T10+T50); recovery, residue, and loss volume; vapor pressure; freezing point; sulfur content; net heat of combustion; copper strip corrosion; oxidation stability (potential gum and lead precipitate); volume change during water reaction; electrical conductivity; and other properties. Avgas fuel is typically tested for its properties using ASTM tests: Motor Octane Number: ASTM D2700 Aviation Lean Rating: ASTM D2700 Performance Number (Super-Charge): ASTM D909 Tetraethyl Lead Content: ASTM D5059 or ASTM D3341 Color: ASTM D2392

Density: ASTM D4052 or ASTM D1298 Distillation: ASTM D86

Vapor Pressure: ASTM D5191 or ASTM D323 or ASTM D5190

Freezing Point: ASTM D2386 Sulfur: ASTM D2622 or ASTM D1266

Net Heat of Combustion (NHC): ASTM D3338 or ASTM D4529 or ASTM D4809

Copper Corrosion: ASTM D130

Oxidation Stability - Potential Gum: ASTM D873 Oxidation Stability - Lead Precipitate: ASTM D873 Water Reaction - Volume change: ASTM D1094 Electrical Conductivity: ASTM D2624 Aviation fuels must have a low vapor pressure in order to avoid problems of vaporization (vapor lock) at low pressures encountered at altitude and for obvious safety reasons. But the vapor pressure must be high enough to ensure that the engine starts easily. The Reid Vapor pressure (RVP) should be in the range of 38kPa to 49kPA. The final distillation point must be fairly low in order to limit the formations of deposits and their harmful consequences (power losses, impaired cooling). These fuels must also possess a sufficient Net Heat of Combustion (NHC) to ensure adequate range of the aircraft. Moreover, as aviation fuels are used in engines providing good performance and frequently operating with a high load, i.e. under conditions close to knocking, this type of fuel is expected to have a very good resistance to spontaneous combustion.

Moreover, for aviation fuel two characteristics are determined which are comparable to octane numbers: one, the MON or motor octane number, relating to operating with a slightly lean mixture (cruising power), the other, the Octane rating. Performance Number or PN, relating to use with a distinctly richer mixture (take-off). With the objective of guaranteeing high octane requirements, at the aviation fuel production stage, an organic lead compound, and more particularly tetraethyllead (TEL), is generally added. Without the TEL added, the MON is typically around 91. As noted above ASTM D910, 100 octane aviation fuel requires a minimum motor octane number (MON) of 99.6. The distillation profile of the high octane unleaded aviation fuel composition should have a T10 of maximum 75°C, T40 of minimum 75°C, T50 of maximum 105°C, and T90 of maximum 135°C.

As in the case of fuels for land vehicles, administrations are tending to lower the lead content, or even to ban this additive, due to it being harmful to health and the environment. Thus, the elimination of lead from the aviation fuel composition is becoming an objective.

Attempts have been made in the past to produce a high octane unleaded aviation fuel that meet most of the ASTM D910 specification for high octane aviation fuel.

In addition to the MON of 99.6, it is also important to not negatively impact the flight range of the aircraft, vapor pressure, and freeze points that meets the aircraft engine start up requirements and continuous operation at high altitude.

US Patent Nos. 9127225, 9388359, 9388357, 9388358, 9120991, 9388356, 9035114 all relate to various unleaded aviation fuel compositions that meet most of the ASTM D910 specification for 100 octane aviation fuel.

US Patent No. 9120991 discloses unleaded aviation fuel compositions comprising toluene, toluidine, alkylate or alkylate blend, branched acetate and isopentane.

US Patent No. 9388356 discloses unleaded aviation fuel compositions comprising toluene, aniline, alkylate or alkylate blend, branched chain alcohol and isopentane.

US Patent No. 9388357 discloses unleaded aviation fuel compositions comprising toluene, aromatic amine component comprising toluidine, alkylate or alkylate blend and isopentane.

US Patent No. 9388358 discloses unleaded aviation fuel compositions comprising toluene, aniline, alkylate or alkylate blend, diethyl carbonate, and isopentane.

US Patent No. 9388359 discloses unleaded aviation fuel compositions comprising toluene, toluidine, alkylate or alkylate blend, diethyl carbonate and isopentane.

US Patent No. 9035114 discloses unleaded aviation fuel compositions comprising toluene, aniline, alkylate or alkylate blend, branched alkyl acetate and isopentane.

US Patent No. 9127225 discloses unleaded aviation fuel compositions comprising toluene, aniline, alkylate or alkylate blend, C4-C5 alcohol, and isopentane.

While the types of compositions disclosed in the above mentioned patent publications may meet most of the ASTM D910 specification for 100 octane aviation fuel, it would be desirable to further improve the octane properties of the fuel composition. It would therefore be desirable to formulate an unleaded aviation fuel which has improved octane properties while still meeting most of the requirements of the ASTM D910 specification. Summary of the Invention

According to the present invention there is provided an unleaded aviation fuel composition having a MON of at least 99.6, sulfur content of less than 0.05 wt%, CHN content of at least 97.2 wt%, less than 2.8 wt% of oxygen content, a T10 of at most 75°C, T40 of at least 75° C, a T50 of at most 105° C, a T90 of at most 135°C, a final boiling point of less than 190°C, an adjusted heat of combustion of at least 43.5 MJ/kg, a vapor pressure in the range of 38 to 49 kPa, comprising from 20 vol.% to 35 vol.% of toluene having a MON of at least 107; from 2 vol.% to 10 vol.% of aniline; from above 30 vol% to 55 vol% of at least one alkylate or alkyate blend having an initial boiling range of from 32°C to 60°C and a final boiling range of from 105°C to 140°C, having T40 of less than 99°C, T50 of less than 100°C, T90 of less than 110°C, the alkylate or alkylate blend comprising isoparaffins from 4 to 9 carbon atoms, 3-20 vol% of C5 isoparaffins, 3-15 vol% of C7 isoparaffins, and 60-90 vol% of C8 isoparaffins, based on the alkylate or alkylate blend, and less than 1 vol% of Cl0+, based on the alkylate or alkylate blend; at least 8 vol% of isopentane in an amount sufficient to reach a vapor pressure in the range of 38 to 49 kPa; from 0.1 vol% to 10 vol%, preferably from 1 vol% to 8 vol%, of a straight chain alkyl acetate having a straight chain alkyl group having 4 to 8 carbon atoms; and from 0.1 vol% to 10 vol%, preferably from 2 vol% to 8 vol%, of a branched chain alcohol having from 4 to 8 carbon atoms, provided that the branched chain does not contain any t-butyl groups; wherein the volume ratio of straight chain alkyl acetate to branched chain alcohol is in the range of 1:3 to 3:1 and wherein the fuel composition contains less than 1 vol% of C8 aromatics.

It has surprisingly been found that the fuel composition of the present invention has improved octane properties.

The features and advantages of the invention will be apparent to those skilled in the art. While numerous changes may be made by those skilled in the art, such changes are within the spirit of the invention.

Brief Description of the Drawings

The drawings illustrate certain aspects of some of the embodiments of the invention, and should not be used to limit or define the invention.

Figure 1 is a graphical representation of the data set out in Tables 6-8.

Detailed Description of the Invention

We have found that the unleaded aviation fuel having the formulation disclosed herein has improved octane properties while meeting most of the ASTM D910 specification for 100 octane aviation fuel.

The unleaded aviation fuel composition of the present invention can be produced by a blend comprising from about 20 vol% to about 35 vol% of high MON toluene, from about 2 vol% to about 10 vol% of aniline, from about 30 vol% to about 55 vol% of at least one alkylate cut or alkylate blend that have certain composition and properties, at least 8 vol% of isopentane, from 0.1 vol% to 10 vol% of a straight chain alkyl acetate and from 0.1 vol% to 10 vol% of a branched chain alcohol.

The volume ratio of straight chain alkyl actetate to branched chain alcohol is in the range of 3:1 to 1:3.

The high octane unleaded aviation fuel of the invention has a MON of greater than 99.6.

Further the unleaded aviation fuel composition contains less than 1 vol%, preferably less than 0.5 vol% of C8 aromatics. It has been found that C8 aromatics such as xylene may have materials compatibility issues, particularly in older aircraft. Further it has been found that unleaded aviation fuel containing C8 aromatics tend to have difficulties meeting the temperature profile (ASTM D86) of D910 specification.

In another embodiment, the unleaded aviation fuel contains no noncyclic ethers. In another embodiment, the unleaded aviation fuel contains no alcohol boiling less than 80°C. Further, the unleaded aviation fuel composition has a benzene content between 0%v and 5%v, preferably less than l%v.

Further, in some embodiments, the volume change of the unleaded aviation fuel tested for water reaction is within +/- 2mL as defined in ASTM D1094.

The high octane unleaded fuel will not contain lead and preferably not contain any other metallic octane boosting lead equivalents. The term "unleaded" is understood to contain less than O.Olg/L of lead. The high octane unleaded aviation fuel will have a sulfur content of less than 0.05 wt%. In some embodiments, it is preferred to have ash content of less than 0.0132g/L (0.05 g/gallon) (ASTM D-482).

According to current ASTM D910 specification, the NHC should be close to or above 43.5mJ/kg. The Net Heat of Combustion value is based on a current low density aviation fuel and does not accurately measure the flight range for higher density aviation fuel. It has been found that for unleaded aviation gasolines that exhibit high densities, the heat of combustion may be adjusted for the higher density of the fuel to more accurately predict the flight range of an aircraft.

There are currently three approved ASTM test methods for the determination of the heat of combustion within the ASTM D910 specification. Only the ASTM D4809 method results in an actual determination of this value through combusting the fuel. The other methods (ASTM D4529 and ASTM D3338) are calculations using values from other physical properties. These methods have all been deemed equivalent within the ASTM D910 specification.

Currently the Net Heat of Combustion for Aviation Fuels (or Specific Energy) is expressed gravimetrically as MJ/kg. Current lead containing aviation gasolines have a relatively low density compared to many alternative unleaded formulations. Fuels of higher density have a lower gravimetric energy content but a higher volumetric energy content (MJ/L).

The higher volumetric energy content allows greater energy to be stored in a fixed volume. Space can be limited in general aviation aircraft and those that have limited fuel tank capacity, or prefer to fly with full tanks, can therefore achieve greater flight range. However, the more dense the fuel, then the greater the increase in weight of fuel carried. This could result in a potential offset of the non-fuel payload of the aircraft. Whilst the relationship of these variables is complex, the formulations in this embodiment have been designed to best meet the requirements of aviation gasoline. Since in part density effects aircraft range, it has been found that a more accurate aircraft range, normally gauged using Heat of Combustion, can be predicted by adjusting for the density of the avgas using the following equation:

HOC* = (HOCv/density)+(% range increase/% payload increase +1) where HOC* is the adjusted Heat of Combustion (MJ/kg), HOCv is the volumetric energy density (MJ/L) obtained from actual Heat of Combustion measurement, density is the fuel density (g/L), % range increase is the percentage increase in aircraft range compared to 100 LL(HOCLL) calculated using HOCv and HOCLL for a fixed fuel volume, and % payload increase is the corresponding percentage increase in payload capacity due to the mass of the fuel.

The adjusted heat of combustion will be at least 43.5MJ/kg, and have a vapor pressure in the range of 38 to 49 kPa. The high octane unleaded fuel composition will further have a freezing point of -58°C or less. Unlike for automobile fuels, for aviation fuel, due to the altitude while the plane is in flight, it is important that the fuel does not cause freezing issues in the air.

Further, the final boiling point of the high octane unleaded fuel composition should be less than 190°C, preferably at most 180°C measured with greater than 98.5% recovery as measured using ASTM D-86. If the recovery level is low, the final boiling point may not be effectively measured for the composition (i.e., higher boiling residual still remaining rather than being measured). The high octane unleaded aviation fuel composition of the invention have a Carbon, Hydrogen, and Nitrogen content (CHN content) of at least 98 wt%, preferably 99 wt%, (and less than 2wt%, preferably lwt% or less of oxygen-content. Suitably, the unleaded aviation fuel composition of the present invention has an aromatics content measured according to ASTM D5134 of greater than 15 vol% to about 35 vol%, preferably in the range of 20 vol% to about 35 vol%, more preferably in the range from 20 vol% to 30 vol%, by weight of the total unleaded aviation fuel composition.

It has been found that the high octane unleaded aviation fuel of the invention not only meets the MON value for 100 octane aviation fuel, but also meets the vapor pressure, adjusted heat of combustion, and freezing point. In addition to MON it is important to meet the vapor pressure, temperature profile, and minimum adjusted heat of combustion for aircraft engine start up and smooth operation of the plane at higher altitude. Preferably the potential gum value is less than 6mg/100mL. In some embodiments, the high octane unleaded aviation fuel has T10 of at most 80°C, a T40 of at least 75°C, a T50 of at most 105°C, a T90 of at most 135°C and a final boiling point of less than 190°C.

It is difficult to meet the demanding specification for unleaded high octane aviation fuel while also having good octane characteristics. It has been found that the unleaded aviation fuel composition of the present invention comprising a certain blend of components in certain amounts serves to address this problem.

Toluene

Toluene occurs naturally at low levels in crude oil and is usually produced in the processes of making gasoline via a catalytic reformer, in an ethylene cracker or making coke from coal. Final separation, either via distillation or solvent extraction, takes place in one of the many available processes for extraction of the BTX aromatics (benzene, toluene and xylene isomers). The toluene used in the invention must be a grade of toluene that have a MON of at least 107 and containing less than lvol% of C8 aromatics. Further, the toluene component preferably has a benzene content between 0%v and 5%v, preferably less than l%v.

For example an aviation reformate is generally a hydrocarbon cut containing at least 70% by weight, ideally at least 85% by weight of toluene, and it also contains C8 aromatics (15 to 50% by weight ethylbenzene, xylenes) and C9 aromatics (5 to 25% by weight propyl benzene, methyl benzenes and trimethylbenzenes). Such reformate has a typical MON value in the range of 102- 106, and it has been found not suitable for use in the present invention.

Toluene is preferably present in the blend in an amount from about 20%v, preferably at least about 25%v, to at most about 40%v, preferably to at most about 35%v, more preferably to at most about 30%v, based on the unleaded aviation fuel composition.

Aniline

Aniline (C6H5NH2) is mainly produced in industry in two steps from benzene. First, benzene is nitrated using a concentrated mixture of nitric acid and sulfuric acid at 50 to 60°C, which gives nitrobenzene. In the second step, the nitrobenzene is hydrogenated, typically at 200- 300°C in presence of various metal catalysts.

As an alternative, aniline is also prepared from phenol and ammonia, the phenol being derived from the cumene process.

In commerce, three brands of aniline are distinguished: aniline oil for blue, which is pure aniline; aniline oil for red, a mixture of equimolecular quantities of aniline and ortho- and para-toluidines; and aniline oil for safranine, which contains aniline and ortho-toluidine, and is obtained from the distillate (echappes) of the fuchsine fusion. Pure aniline, otherwise known as aniline oil for blue is desired for high octane unleaded avgas. Aniline is preferably present in the blend in an amount from about 2%v, preferably at least about 3%v, most preferably at least about 4%v to at most about 10%v, preferably to at most about 7%v, more preferably to at most about 6%v, based on the unleaded aviation fuel composition.

Alkylate and Alkyate Blend

The term alkylate typically refers to branched-chain paraffin. The branched-chain paraffin typically is derived from the reaction of isoparaffin with olefin. Various grades of branched chain isoparaffins and mixtures are available. The grade is identified by the range of the number of carbon atoms per molecule, the average molecular weight of the molecules, and the boiling point range of the alkylate. It has been found that a certain cut of alkylate stream and its blend with isoparaffins such as isooctane is desirable to obtain or provide the high octane unleaded aviation fuel of the invention. These alkylate or alkylate blend can be obtained by distilling or taking a cut of standard alkylates available in the industry. It is optionally blended with isooctane. The alkylate or alkyate blend have an initial boiling range of from about 32°C to about 60°C and a final boiling range of from about 105°C to about 140°C, preferably to about 135°C, more preferably to about 130°C, most preferably to about 125°C, having T40 of less than 99°C, preferably at most 98C, T50 of less than 100°C, T90 of less than 110°C, preferably at most 108°C, the alkylate or alkylate blend comprising isoparaffins from 4 to 9 carbon atoms, about 3-20 vol% of C5 isoparaffins, based on the alkylate or alkylate blend, about 3-15 vol% of C7 isoparaffins, based on the alkylate or alkylate blend, and about 60-90 vol% of C8 isoparaffins, based on the alkylate or alkylate blend, and less than 1 vol% of C10+, preferably less than 0.1 vol%, based on the alkylate or alkylate blend. Alkylate or alkylate blend is preferably present in the unleaded aviation fuel composition in an amount from about 30%v, preferably at least about 32%v, most preferably at least about 35%v to at most about 55%v, preferably to at most about 49%v, more preferably to at most about 47%v, based on the unleaded aviation fuel composition.

Isopentane

Isopentane is present in an amount of at least 8 vol% in an amount sufficient to reach a vapor pressure in the range of 38 to 49 kPa. The alkylate or alkylate blend also contains C5 isoparaffins so this amount will typically vary between 5 vol% and 25 vol% depending on the C5 content of the alkylate or alkylate blend. Isopentane should be present in an amount to reach a vapor pressure in the range of 38 to 49 kPa to meet aviation standard. The total isopentane content in the unleaded aviation fuel composition is typically in the range of 10 vol% to 26 vol%, preferably in the range of 12% to 18% by volume, based on the unleaded aviation fuel composition.

Alkyl Acetate

The unleaded aviation fuel composition contains a straight chain alkyl acetate having a straight chain alkyl group having 4 to 8 carbon atoms as a co-solvent. Straight chain alkyl acetates having from 4 to 8 carbon atoms includes n-butyl acetate, n-pentyl acetate, n-hexyl acetate, n-heptyl acetate and n-octyl acetate.

Preferably the straight chain alkyl group has from 4 to 6 carbon atoms, more preferably 4 or 5 carbon atoms, and especially 4 carbon atoms. It has been found that the larger the co-solvent molecule, the higher the density of the fuel, which may lead to soot issues associated with the use of higher density fuels in piston engines. In an especially preferred embodiment, the co-solvent for use herein is n-butyl acetate.

The unleaded aviation fuels containing aromatic amines tend to be significantly more polar in nature than traditional aviation gasoline base fuels. As a result, they have poor solubility in the fuels at low temperatures, which can dramatically increase the freeze points of the fuels. Consider for example an aviation gasoline base fuel comprising 10% v/v isopentane, 70% v/v light alkylate and 20% v/v toluene. This blend has a MON of around 90 to 93 and a freeze point (ASTM D2386) of less than -76°C. The addition of 6% w/w (approximately 4% v/v) of the aromatic amine aniline increases the MON to 96.4. At the same time, however, the freeze point of the resultant blend (again measured by ASTM D2386) increases to -12.4°C. The current standard specification for aviation gasoline, as defined in ASTM D910, stipulates a maximum freeze point of -58°C. Therefore, simply replacing TEL with a relatively large amount of an alternative aromatic octane booster would not be a viable solution for an unleaded aviation gasoline fuel. It has been found that straight chain alkyl acetates used herein dramatically decrease the freezing point of the unleaded aviation fuel to meet the current ASTM D910 standard for aviation fuel. The straight chain alkyl acetate is present in an amount from 0.1 vol%, to 10 vol%, preferably from 1 vol% to 8 vol%, more preferably from 3 vol% to 6 vol%, even more preferably from 4 vol% to 6 vol%, based on the unleaded aviation fuel composition.

The straight chain alkyl acetate is useful in combination with the branched alkyl alcohol for providing improved octane characteristics. In a preferred embodiment herein, the volume ratio of the straight chain alkyl actetate to branched chain alkyl alcohol is in the range of 2:1 to 1:2, most preferably at a ratio of 1:1.

Preferably the water reaction volume change is within +/- 2ml for aviation fuel. Water reaction volume change is large for ethanol that makes ethanol not suitable for aviation gasoline.

Branched Chain Alcohol

The unleaded aviation fuel composition contains a branched chain alcohol having from 4 to 8 carbon atoms, preferably from 4 to 6 carbon atoms, as an additional co solvent, provided that the branched chain does not include t-butyl groups. Suitable branched chain alcohols as an additional co-solvent may be, for example, iso butyl alcohol, iso-pentyl alcohol, iso-hexyl alcohol, iso-heptyl alcohol, iso-octyl alcohol, and mixtures thereof.

The branched chain alcohol is present in an amount from 0.1 vol%, to 10 vol%, preferably from 2 vol% to 8vol%, more preferably from 3 vol% to 6 vol%, even more preferably from 4 vol% to 6 vol%, based on the unleaded aviation fuel composition. A preferred branched chain alcohol for use herein is isobutyl alcohol (IBA).

In an especially preferred embodiment herein, the unleaded aviation fuel composition comprises 4 vol% of branched chain alcohol and 4 vol% of straight chain acetate.

Blending

For the preparation of the high octane unleaded aviation gasoline, the blending can be in any order as long as they are mixed sufficiently. It is preferable to blend the toluene, and alkylate blend together, followed by the isopentane, isobutane, and then the straight chain alkyl acetate, the branched chain alcohol and the aniline (in that order) and to mix the blend for about 2 hours. This order of addition helps to prevent the aniline dropping out of solution.

In order to satisfy other requirements, the unleaded aviation fuel according to the invention may contain one or more additives which a person skilled in the art may choose to add from standard additives used in aviation fuel. There should be mentioned, but in non-limiting manner, additives such as antioxidants, anti-icing agents, antistatic additives, corrosion inhibitors, dyes and their mixtures.

According to another embodiment of the present invention a method for operating an aircraft engine, and/or an aircraft which is driven by such an engine is provided, which method involves introducing into a combustion region of the engine and the high octane unleaded aviation gasoline fuel formulation described herein. The aircraft engine is suitably a spark ignition piston-driven engine. A piston-driven aircraft engine may for example be of the inline, rotary, V-type, radial or horizontally-opposed type.

The unleaded aviation gasoline fuel formulation described herein provides improvements in octane properties. Hence, according to another embodiment of the present invention there is provided a use of an unleaded aviation fuel composition having a MON of at least 99.6, sulfur content of less than 0.05 wt%, CHN content of at least 97.2 wt%, less than 2.8 wt% of oxygen content, a T10 of at most 75°C, T40 of at least 75° C, a T50 of at most 105° C, a T90 of at most 135°C, a final boiling point of less than 190°C, an adjusted heat of combustion of at least 43.5 MJ/kg, a vapor pressure in the range of 38 to 49 kPa, comprising from 20 vol.% to 35 vol.% of toluene having a MON of at least 107; from 2 vol.% to 10 vol.% of aniline; from above 30 vol% to 55 vol% of at least one alkylate or alkyate blend having an initial boiling range of from 32°C to 60°C and a final boiling range of from 105°C to 140°C, having T40 of less than 99°C , T50 of less than 100°C, T90 of less than 110°C, the alkylate or alkylate blend comprising isoparaffins from 4 to 9 carbon atoms, 3-20 vol% of C5 isoparaffins, 3-15 vol% of C7 isoparaffins, and 60-90 vol% of C8 isoparaffins, based on the alkylate or alkylate blend, and less than 1 vol% of Cl0+, based on the alkylate or alkylate blend; at least 8 vol% of isopentane in an amount sufficient to reach a vapor pressure in the range of 38 to 49 kPa; from 0.1 vol% to 10 vol%, preferably from 2 vol% to 6 vol%, of a straight chain alkyl acetate having a straight chain alkyl group having 4 to 8 carbon atoms; and from 0.1 vol% to 10 vol%, preferably from 2 vol% to 8 vol%, of a branched chain alcohol having from 4 to 8 carbon atoms, provided that the branched chain does not contain any t-butyl groups; wherein the volume ratio of straight chain alkyl actetate to branched chain alcohol is in the range of 3:1 to 1:3 and wherein the fuel composition contains less than 1 vol% of C8 aromatics for the purpose of providing improved octane properties. As used herein, the term 'improved octane properties' means that the unleaded aviation fuel composition has an increased MON value. MON is used for measuring the antiknock performance of spark-ignition engine fuels. It is utilized to determine, by correlation equation, the Aviation method octane number or performance number. The higher the octane number, the more compression the fuel can withstand before detonating. Fuels with higher octane allow higher performance of gasoline engines.

In the context of this aspect of the invention, the term 'improved octane properties' embraces any degree of improvement in octane properties. The improvement in octane properties may be of the order of 0.5% or more, preferably 1% or more, more preferably 5% or more, and especially 10% or more compared to the octane properties exhibited by an analogous fuel formulation which does not contain the same blend of components in the specified amounts as the fuel formulations of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of examples herein described in detail. It should be understood, that the detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. The present invention will be illustrated by the following illustrative embodiment, which is provided for illustration only and is not to be construed as limiting the claimed invention in any way. Examples Test Methods The following test methods were used for the measurement of the aviation fuels.

Motor Octane Number: ASTM D2700 Tetraethyl Lead Content: ASTM D5059 Density: ASTM D4052 Distillation: ASTM D86

Vapor Pressure: ASTM D323 Freezing Point: ASTM D2386 Sulfur: ASTM D2622

Net Heat of Combustion (NHC): ASTM D3338 Copper Corrosion: ASTM D130

Oxidation Stability - Potential Gum: ASTM D873 Oxidation Stability - Lead Precipitate: ASTM D873 Water Reaction - Volume change: ASTM D1094 Detail Hydrocarbon Analysis (ASTM 5134) Examples 1-15

Aviation fuel compositions of the invention were prepared by blending a base fuel, aniline and co-solvent. The base fuel was a blend of 20%v isopentane, 52%v light aviation alkylate and 28%v toluene and had the properties shown in Table 1 below.

Table 1 (base fuel properties)

In the aviation fuel compositions prepared, the hydrocarbon and aniline part of the fuel composition remained constant while the co-solvent was varied. The co-solvent consisted of different alkyl acetates with isobutyl alcohol in different ratios. Properties of the different acetates and alcohol used in these are blends are summarized in Table 2 below. Table 2

The acetate and isobutanol were mixed and then added to the blend as one component. The fuel compositions were prepared by blending base fuel, co-solvent and aniline, in that order, in amounts set out in Tables 3-6 below. After mixing, each blend was mechanically stirred for 2 to 3 minutes and was then sent for MON (ASTM D2700) and Distillation (ASTM D86) testing.

Table 3

Table 4

Table 5

The results of the MON and the Distillation testing are set out in Table 6-8 below.

24

Table 6

25

Table 7

26

Table 8

Based on the data shown in Tables 6-8, it can be seen that mixing the different acetates with isopropyl alcohol has an effect on the MON. While the isobutyl acetate and t-butyl acetate blends have shown a decrease in MON as isobutyl alohol is added, the n-butyl acetate blend has shown an increase in MON as the cosolvent mixture reaches a 50/50 blend ratio and drops again at a 25/75 ratio n-butyl acetate/isobutyl alcohol. This can also be seen in Figure 1 which is a graphical representation of the data set out in Tables 6-8.

Examples 16-20

Further fuel compositions were prepared having the compositions set out in Table 9 below. The same preparation method was used as in Examples 1-15 above. Various branched chain alcohols were used as a co-solvent as indicated in combination with n-butyl acetate. The alcohols used were isobutyl alcohol (IBA), isopentyl (IPA), isohexyl alcohol (IHexA), isoheptyl (IHepA), isooctyl alcohol (IOA). The MON of Examples 16-20 are shown in Table 9 below.

Table 9

N.M. = Not measured

Claims

C LA IM S

1. An unleaded aviation fuel composition having a MON of at least 99.6, sulfur content of less than 0.05 wt%, CHN content of at least 97.2 wt%, less than 2.8 wt% of oxygen content, a T10 of at most 75°C, T40 of at least 75° C, a T50 of at most 105° C, a T90 of at most 135°C, a final boiling point of less than 190°C, an adjusted heat of combustion of at least 43.5 MJ/kg, a vapor pressure in the range of 38 to 49 kPa, comprising from 20 vol.% to 35 vol.% of toluene having a MON of at least 107; from 2 vol.% to 10 vol.% of aniline; from above 30 vol% to 55 vol% of at least one alkylate or alkyate blend having an initial boiling range of from 32°C to 60°C and a final boiling range of from 105°C to 140°C, having T40 of less than 99°C, T50 of less than 100°C, T90 of less than 110°C, the alkylate or alkylate blend comprising isoparaffins from 4 to 9 carbon atoms, 3-20 vol% of C5 isoparaffins, 3-15 vol% of C7 isoparaffins, and 60-90 vol% of C8 isoparaffins, based on the alkylate or alkylate blend, and less than 1 vol% of Cl0+, based on the alkylate or alkylate blend; at least 8 vol% of isopentane in an amount sufficient to reach a vapor pressure in the range of 38 to 49 kPa; from 0.1 vol% to 10 vol%, preferably from 1 vol% to 8 vol%, of a straight chain alkyl acetate having a straight chain alkyl group having 4 to 8 carbon atoms; and from 0.1 vol% to 10 vol%, preferably from 2 vol% to 8 vol%, of a branched chain alcohol having from 4 to 8 carbon atoms, provided that the branched chain does not contain any t-butyl groups; wherein the volume ratio of straight chain alkyl actetate to branched chain alcohol is in the range of 3:1 to 1:3; and wherein the fuel composition contains less than 1 vol% of C8 aromatics.

2. An unleaded aviation fuel composition according to Claim 1 wherein the volume ratio of straight chain alkyl acetate to branched chain alcohol is in the range of from 2:1 to 1:2.

3. An unleaded aviation fuel composition according to claim 1 or 2, wherein the total content of isopentane, in the fuel composition is from 10 vol.% to 20 vol%.

4. An unleaded aviation fuel composition according to claims 1 to 3, further comprising an aviation fuel additive.

5. An unleaded aviation fuel composition according to claims 1 to 4, having a freezing point of less than -58 °C.

6. An unleaded aviation fuel composition of claims 1 to

5, wherein the alkylate or alkylate blend have a C10+ content of less than 0.1 vol% based on the alkylate or alkylate blend.

7. An unleaded aviation fuel composition of claims 1 to

6, wherein the straight chain alkyl acetate is n-butyl acetate.

8. An unleaded aviation fuel composition according to any of Claims 1 to 7 wherein the branched alcohol is isobutyl alcohol.

9. An unleaded aviation fuel composition of claims 1 to 8, having a final boiling point of at most 180°C.

10. Use of an unleaded aviation fuel composition having a MON of at least 99.6, sulfur content of less than 0.05 wt%, CHN content of at least 97.2 wt%, less than 2.8 wt% of oxygen content, a T10 of at most 75°C, T40 of at least 75° C, a T50 of at most 105° C, a T90 of at most 135°C, a final boiling point of less than 190°C, an adjusted heat of combustion of at least 43.5 MJ/kg, a vapor pressure in the range of 38 to 49 kPa, comprising from 20 vol.% to 35 vol.% of toluene having a MON of at least 107; from 2 vol.% to 10 vol.% of aniline; from above 30 vol% to 55 vol% of at least one alkylate or alkyate blend having an initial boiling range of from 32°C to 60°C and a final boiling range of from 105°C to 140°C, having T40 of less than 99°C, T50 of less than 100°C, T90 of less than 110°C, the alkylate or alkylate blend comprising isoparaffins from 4 to 9 carbon atoms, 3-20 vol% of C5 isoparaffins, 3-15 vol% of C7 isoparaffins, and 60-90 vol% of C8 isoparaffins, based on the alkylate or alkylate blend, and less than 1 vol% of Cl0+, based on the alkylate or alkylate blend; at least 8 vol% of isopentane in an amount sufficient to reach a vapor pressure in the range of 38 to 49 kPa; from 0.1 vol% to 10 vol%, preferably from 1 vol% to 8 vol%, of a straight chain alkyl acetate having a straight chain alkyl group having 4 to 8 carbon atoms; and from 0.1 vol% to 10 vol%, preferably from 2 vol% to 8 vol%, of a branched chain alcohol having from 4 to 8 carbon atoms, provided that the branched chain does not contain any t-butyl groups; wherein the volume ratio of straight chain alkyl actetate to branched chain alcohol is in the range of 3:1 to 1:3; and wherein the fuel composition contains less than 1 vol% of C8 aromatics, for the purpose of providing improved octane properties.