SYNERGISTIC ANTI-KNOCK ADDITIVE FOR FUELS AND GASOLINE COMPOSITION COMPRISING IT

Abstract

The present invention relates to a synergistic antiknock fuel additive comprising a mixture of N-methyl-p-anisidine and m-toluidine in a volume ratio from 45:55 to 95:5; to the use of said fuel additive to increase the octane rating of a gasoline; and to a gasoline fuel composition comprising said fuel additive.

Description

SYNERGISTIC ANTI-KNOCK ADDITIVE FOR FUELS AND GASOLINE COMPOSITION THAT INCLUDES IT Field of the invention The present invention relates to the petrochemical industry. More specifically, the present invention relates to a synergistic antiknock fuel additive comprising a mixture of N-methyl-p-anisidine and m-toluidine; to the use of said additive to increase the octane rating of gasoline; and to a gasoline fuel composition comprising said additive. Background of the invention Gasoline with a higher octane rating allows for higher compression ratios in combustion engines, which improves engine efficiency and contributes to a reduction in greenhouse gas (GHG) emissions from transportation. Most conventional vehicles in developed markets use a knock-detection device (KD), which allows for the monitoring of engine parameters based on gasoline quality to prevent knocking; depending on the gasoline octane rating and operating speed, this monitoring can lead to a loss of efficiency (higher fuel consumption) or reduced performance. Optimizing the operating speeds of most high-performance vehicles requires the use of gasoline with a higher octane rating than conventional gasoline. The most technologically and economically advantageous way to produce high-octane gasoline is through the use of anti-knock additives. The first anti-knock additives or octane boosters used on a large scale were tetraalkyl lead additives, which are now banned due to their toxicity and impact on the vehicle's catalytic converter. Oxygenated compounds can improve the octane rating of gasoline, the most well-known being methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE). Their main drawback is the need to use high quantities, which limits the use of cheaper lighter components in gasoline. Their partial solubility in water has limited the use of ethers in several regions due to the risk of contamination of groundwater reservoirs. Low-aliphatic alcohols, such as methanol, ethanol, isopropanol, tert-butyl alcohol, or isobutanol, are permitted for use in automotive gasoline. Their octane-boosting capacity is lower than that of ethers, and their side effects are more noticeable, since they significantly increase the vapor pressure of gasoline and are highly soluble in water.There is a long list of oxygenates that have been evaluated as octane boosters, including ketones, esters, etc. PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ furans and carbonates, but their use has not been successful due to their cost, their harmful effects or their impact on health and the environment. The use of highly efficient octane boosters that can be used at lower concentrations has been the focus of several studies. Metallic antiknock agents that are alternatives to lead compounds, such as MMT or ferrocene, are highly efficient but tend to form ash deposits after combustion and their use is prohibited in most automotive gasoline specifications. Ashless octane boosters are preferred over metallic ones. Amines, nitrosamines, hydrazines, pyridines, and quinolines have been the subject of several reports. The use of some aromatic amines, and particularly anilines, to increase the octane rating of base gasoline has been widely reported due to their very high efficiency as octane boosters. Aromatic amines are generally more efficient at increasing octane rating than oxygenated additives. However, they are usually more expensive and are sometimes highly toxic (e.g., aniline and o-toluidine). EP 2014643 A1 and WO 2008 / 076759 disclose the use of N-methyl-p-anisidine (NMPA) to increase the octane rating of hydrocarbon fuels. This aromatic amine exhibits highly effective antiknock properties. However, NMPA is very expensive and is a crystalline solid with a melting point of 33-36 °C, potentially leading to crystallization at low ambient temperatures. Handling this product therefore requires costly investments or the use of solvents, which reduces the main advantage of aniline compounds by increasing the dosage and also impacts gasoline blend-limiting properties such as volatility and density. One option to reduce this loss of NMPA's benefits is to improve its handling by combining it with solvents that can improve octane rating. However, when using ethers such as ETBE or alcohols such as ethanol as solvents, high dilution is required, which significantly increases the final additive dosage and, therefore, has a significant impact on the cost and final properties of the gasoline. Document RU 2633357 C1 discloses a multifunctional additive for automotive gasoline comprising a mixture of aliphatic alcohols, antioxidants, detergents and aromatic amines selected from m-toluidine and / or NMPA and / or 2,4-xylidine. This document does not mention any synergistic antiknock effect for any of the combinations of said aromatic amines. In fact, the ratio of NMPA to m-toluidine disclosed in this document is very far from the ratio found in the present invention for PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ provide a synergistic antiknock effect. This document discloses an additive comprising a very low content of NMPA mixed with m-toluidine: 20.5% w / w NMPA and 40.0% w / w m-toluidine, i.e. 32% v / v NMPA in the NMPA / m-toluidine mixture. This amount of NMPA is far from the minimum NMPA content of about 45% v / v that the inventors of the present invention have found to be required in others to provide a synergistic effect when combined with m-toluidine (considering the uncertainty of the results). Octane rating improvements must be tailored to new automotive engines. Anti-knock properties have been linked to the combination of both RON (Research Octane Rating, ASTM D2699) and MON (Motor Octane Rating, ASTM 2700). All specifications require a minimum value for both, and octane improvers have been evaluated for their ability to increase both. However, it is well known that the anti-knock properties of new engines are more dependent on high RON values, while the influence of MON is less relevant. There is still a need to develop new anti-knock agents with excellent anti-knock performance, which are non-toxic and economical. Summary of the invention The invention is based on the discovery that the antiknock agents m-toluidine (m-tol) and N-methyl-p-anisidine (NMPA) provide a synergistic increase in the octane rating of a gasoline when blended in certain proportions. That is, the blend induces a combined octane rating increase greater than the sum of the octane rating increases induced individually by each of these agents at the same concentration in the same gasoline. Thus, the present invention provides an antiknock additive having very high efficiency. Furthermore, due to the synergistic interaction of m-toluidine and NMPA, the desired octane rating can be achieved with a lower amount of NMPA when blended with the much less expensive antiknock agent m-toluidine, thereby resulting in a more economical additive than when NMPA is used alone. Therefore, the invention provides an anti-knock agent with excellent anti-knock performance and reduced cost, and which is metal-free. PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ Furthermore, m-toluidine has been found to be an excellent solvent for NMPA, which is solid at room temperature. m-toluidine keeps NMPA solubilized at low temperatures, preventing NMPA crystallization even at much higher NMPA concentrations than other typical solvents, such as ethanol, ethyl tert-butyl ether, ethylbenzene, or 2-butoxyethanol. The result is a stable composition over a wide temperature range, ensuring no loss of antiknock properties during extended storage and transportation. This improved low-temperature performance enables its transportation, storage, and use in liquid form under a wide range of climatic conditions. The inventors have also surprisingly found that m-toluidine reduces the degradation of vehicle engine lubricants contaminated with other anilines, such as NMPA. This contamination occurs in used engine lubricants when aniline antiknock agents are used. Some vehicle manufacturers have opposed the use of this type of octane booster due to the incompatibility of anilines with engine lubricant. CEN Technical Report TR 17491 provides information on aniline and aniline derivatives when used as blending components in unleaded fuel. Specifically, some anilines are addressed: aniline, N-methylaniline, N-ethylaniline, and N,N-dimethylaniline. These compounds are reported to reduce the durability of engine lubricants in rig tests under accelerated conditions.Now, the inventors have found that m-toluidine improves the compatibility of NMPA with engine lubricant, compared to using NMPA alone. The engine lubricant degradation observed at high concentrations of NMPA is significantly reduced when mixed with m-toluidine, leading to fewer changes in engine lubricant viscosity and fewer adherent deposits at the end of the test. Finally, this anti-knock additive shows low toxicity and therefore does not need to be added to gasoline at the refinery; rather, it can be added directly to gasoline at the gas station (with appropriate safety validation procedures and equipment). Thus, in a first aspect, the invention is directed to a fuel additive comprising a mixture of N-methyl-p-anisidine and m-toluidine in a volume ratio of 45:55 to 95:5. In a second aspect, the invention is directed to a gasoline fuel composition comprising a fuel additive as defined in the first aspect and a gasoline base fuel. In a third aspect, the present invention is directed to the use of a fuel additive as defined in the first aspect of the invention for improving the anti-knock properties of PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ a gasoline. In a further aspect, the present invention is directed to a method for improving the research octane rating (RON) of a gasoline, comprising adding a fuel additive as defined in the first aspect of the invention to the gasoline. Brief description of the figures Figure 1 represents the actual research octane number (RON) values ​​of a high octane gasoline composition comprising 1 vol % blends of m-toluidine and NMPA in different proportions, compared to the predicted RON values ​​for such compositions. Figure 2 represents the actual octane number values ​​(RON and MON in combination) of a gasoline composition comprising between 0.2 and 2% by volume of mixtures of mtoluidine and NMPA in different proportions, for a given experimental value of RON. Figure 3 represents the actual laboratory measured octane number (RON) values ​​of a low octane gasoline composition comprising 1 vol % blends of m-toluidine and NMPA in different proportions, compared to the predicted RON values ​​for such compositions. Figure 4 represents the cloud point of mixtures of NMPA with various solvents at different NMPA concentrations. Figure 5 shows photographs of drained flasks showing the adherent insoluble materials in a laboratory simulated spent lubricant (modified CEO L-09 test) with conventional gasoline dilution and severe octane enhancer contamination, for compositions comprising different amounts of an antiknock additive comprising either NMPA alone (top row) or a 50:50 mixture of m-toluidine and NMPA (bottom row). Detailed description of the invention Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by one of ordinary skill in the art to which this disclosure pertains. As used herein, the singular forms “a,” “an,” and “the” include plural references, unless the context clearly dictates otherwise. PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ The octane rating can be determined using either the "research" (RON) method or the "motor" (MON) method. In this document, the octane rating refers to the research octane rating (RON) unless clearly stated otherwise. The most common type of octane rating is the Research Octane Rating (RON), and gasolines are typically rated based on this. RON is determined by passing fuel through a test engine at varying compression ratio under controlled conditions and comparing the results with those of conventional blends of iodine-octane improver, n-heptane, and octane. RON can be measured according to ASTM D2699. Motor Octane Rating (MON) complements the evaluation of antiknock performance and can be measured according to ASTM 2700. The combination of RON and MON defines the octane rating, and the relative impact of each depends on the engine type. Higher RON values ​​result in better antiknock performance in all types of engines, especially in modern engines where MON has less influence. The terms “octane booster,” “anti-knock agent,” or “anti-knock additive” are used interchangeably herein to refer to compounds or mixtures of compounds that increase the octane rating of a fuel composition. All embodiments and definitions disclosed in the context of one aspect of the invention are also applicable to the other aspects of the invention. In a first aspect, the invention relates to a fuel additive comprising a mixture of N-methyl-p-anisidine and m-toluidine in a volume ratio range of from 45:55 to 95:5. In a preferred embodiment of the invention, the additive comprises a mixture of N-methyl-p-anisidine and m-toluidine in a volume ratio range of from 50:50 to 90:10. Preferably, the fuel additive comprises a mixture of N-methyl-p-anisidine and m-toluidine in a volume ratio range of 45:55 to 90:10; preferably 45:55 to 85:15; more preferably 50:50 to 75:25. In one embodiment, the fuel additive of the invention consists of N-methyl-p-anisidine and m-toluidine in the proportions indicated above. In a particular embodiment, the fuel additive further comprises an antioxidant, preferably in an amount equal to or less than 1000 mg of antioxidant per Kg of aniline mixture (i.e., per Kg of the mixture of N-methyl-p-anisidine plus m-toluidine), such as from pqqi? Ln / zznz / e / γAΛA 100 mg / kg to 1000 mg / kg, preferably from 200 mg / kg to 1000 mg / kg; more preferably from 500 mg / kg to 1000 mg / kg. The inventors have observed that the use of thiourea-based or hydrazine-based antioxidants provides very high stability to the gasoline fuel composition comprising the fuel additive of the invention. Therefore, in a particular embodiment, the fuel additive of the invention comprises an antioxidant selected from thiourea-based antioxidants, hydrazine-based antioxidants, and mixtures thereof, preferably in an amount of 100-1000 mg, more preferably 500-1000 mg, per kg of aniline mixture. In a preferred embodiment, the antioxidant in the fuel additive is a thiourea-based antioxidant. Examples of thiourea-based antioxidants include, but are not limited to, compounds represented by the formula R1R2NC(=S)NR3R4, wherein R1, R2, R3, and R4 are independently selected from hydrogen, C1-4 alkyl optionally substituted with (C1-4 alkyl)amino, di(C1-4 alkyl)amino or alkoxy; C3.4 cycloalkyl; and C6 aryl; or R1 and R3 may together form a C2-4 alkylene; or R1 and R2 or R3 and R4 may form a C3-5 alkylene optionally interrupted by an oxygen or nitrogen atom.Los ejemplos de antioxidantes basados ​​en tiourea incluyen tiourea, 1,3-ethylenethiolurea, trimethyltiourea, tributyltiourea, 1,3-diethyltiourea, 1,3dibutyltiourea, 1,3-bis(dimetilam¡noprop¡l)-2-t¡ourea, N-phenyltiourea, 1-metoxipropyl-3-butyl-2thiourea, 1-dimet¡lam¡noprop¡l-3-but¡l-2-thiourea, 1-metoxipropyl-3-c¡clohex¡l-2-t¡ourea, 1dimetilaminopropyl-3-phen¡l-2-thiourea, 1 -metoxipropyl-3,3-d¡but¡l-2-t¡ourea, 1 -dimethylaminopropyl3,3-diisoprop¡l-2-thiourea, 1 -diet¡lam¡nopropyl-3-methyl-3-c¡clohexyl-2-thiourea, 1 -methoxypropyl-3-phenyl3-cyclohexyl-2-t¡ourea, 1 -methox¡prop¡l-3-oxid¡et¡len-2-t¡ourea, 1 -n-butyl-3-oxid¡et¡len-2-t¡ourea, 1 diet¡lam¡noprop¡l-3-ox¡d¡d¡len-2-t¡ourea. Preferably, the thiourea-based antioxidant is 1,3ethylenethiourea. Examples of hydrazine-based antioxidants include, but are not limited to, hydrazine, hydrazine hydrate, 1,1-dimethylhydrazine, 1,2-diphenylhydrazine, acetohydrazide, benzohydrazide, cyclohexanecarbohydrazide, adipic acid dihydrazide, sebacic acid dihydrazide, dodecanedioic acid dihydrazide, isophthalic acid dihydrazide, propionic acid hydrazide, salicylic acid hydrazide, 3-hydroxy-2-naphthoic acid hydrazide, benzophenone hydrazone, aminopolyacrylamide, N,N'-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl]hydrazine, isopropylhydrazine sulfate, tert-butylhydrazine sulfate, decamethylenedicarboxylic acid bis(N'-salicyloylhydrazide), isophthalic acid bis(2-phenoxypropionylhydrazide), N-formyl-N'salicyloylhydrazine, N,N'-bis[beta(3,5-d¡-tert-but¡l-4-hydroxyphen¡l)propan-acyl]hydrazine, N,N'-bis(3,5di-tert-but¡l-4-hydroxy¡phen¡lpropíon¡l)hydrazine and N-salicyloyl-N'-aldehydohydrazine.Preferably, the hydrazine-based antioxidant is hydrazine or a hydrate thereof. PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ In one embodiment, the fuel additive comprises N-methyl-p-anisidine, m-toluidine, and an antioxidant, preferably an antioxidant selected from thiourea-based antioxidant, hydrazine-based antioxidant, and mixtures thereof. In a further embodiment, the fuel additive consists of N-methyl-p-anisidine, m-toluidine, and an antioxidant, preferably a thiourea-based antioxidant, a hydrazine-based antioxidant, or mixtures thereof. In a preferred embodiment, the fuel additive comprises at least 90% by weight or at least 95% by weight of the mixture of N-methyl-p-anisidine and m-toluidine, based on the total weight of the fuel additive. In a further preferred embodiment, the fuel additive comprises at least 99% by weight or at least 99.5% by weight of the mixture of N-methyl-p-anisidine and m-toluidine, based on the total weight of the fuel additive. In one embodiment, the fuel additive of the invention does not comprise an alcohol, such as a C1-4 alcohol, including methanol, ethanol, isopropyl alcohol, isobutyl alcohol, tert-butyl alcohol. In a further embodiment, the fuel additive does not comprise isopropyl alcohol. The fuel additive of the invention provides a synergistic improvement of the research octane number and is therefore a synergistic anti-knock fuel additive. In a second aspect, the invention is directed to a gasoline fuel composition comprising the fuel additive of the invention and a gasoline base fuel. In one embodiment, the fuel additive is present in the gasoline fuel composition in an amount of from 0.2% to 5.0% by volume relative to the volume of the gasoline fuel composition; preferably from 0.5% to 3.0% by volume. In a more preferred embodiment, the fuel additive is present in the gasoline fuel composition in an amount of from 0.5% to 2.0% by volume relative to the volume of the gasoline fuel composition, or from 0.5% to 1.5% by volume; even more preferably from 0.5% to 1.0% by volume. The gasoline fuel composition may comprise from 0.2% to 5.0% by volume relative to the volume of the gasoline fuel composition, PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ preferably from 0.5% to 3.0% by volume, of a mixture of m-toluidine and NMPA, wherein the volume ratio of NMPA to m-toluidine is from 45:55 to 95:5, preferably from 45:55 to 90:10 or from 45:55 to 85:15, more preferably from 50:50 to 75:25. In one embodiment, the gasoline fuel composition may comprise from 0.5% to 2.0% by volume relative to the volume of the gasoline fuel composition, preferably from 0.5% to 1.5% by volume, of a mixture of m-toluidine and NMPA, wherein the volume ratio of NMPA to m-toluidine is from 45:55 to 95:5, preferably from 45:55 to 90:10 or from 45:55 to 85:15, more preferably from 50:50 to 75:25. In one embodiment, the volume ratio of NMPA to m-toluidine in the various aspects and embodiments of the invention is from 45:55 to 90:10. In another embodiment, the volume ratio of NMPA to m-toluidine is from 50:50 to 90:10. In a further embodiment, the volume ratio of NMPA to m-toluidine is from 45:55 to 85:15. The amount of gasoline base fuel in the gasoline composition is typically at least 95% by volume based on the volume of the gasoline fuel composition, or at least 97% by volume, and preferably at least 98% or 99% by volume. In one embodiment, the amount of gasoline base fuel in the gasoline composition is from 97% to 99.5% by volume based on the volume of the gasoline fuel composition, preferably from 98% to 99.5% by volume. The gasoline base fuel may be any gasoline suitable for use in a spark ignition type internal combustion engine known in the art, such as in automobile engines, as well as in other types of engines, such as off-road and aircraft engines. Gasoline-based fuels typically comprise a mixture of hydrocarbons in the boiling range of gasoline. Suitable hydrocarbons in the boiling range of gasoline are hydrocarbon mixtures having a boiling range of from about 25°C to about 232°C and comprising mixtures of saturated hydrocarbons, olefinic hydrocarbons, and aromatic hydrocarbons. why? Ln / zznz / e / YiAi In one embodiment, the gasoline base fuel comprises a mixture of saturated hydrocarbons, olefinic hydrocarbons and aromatic hydrocarbons and, optionally, oxygenated hydrocarbons. Typically, the saturated hydrocarbon content in the gasoline-based fuel ranges from 30% to 90% by volume based on the volume of the gasoline-based fuel; preferably from 40% to 90% by volume; more preferably from 40% to 80% by volume. Typically, the olefinic hydrocarbon content in the gasoline-based fuel ranges from 2% to 30% by volume based on the volume of the gasoline-based fuel; preferably from 5% to 30% by volume; more preferably from 5% to 20% by volume. Typically, the aromatic hydrocarbon content in the gasoline base fuel ranges from 10% to 60% by volume based on the volume of the gasoline base fuel; preferably from 15% to 45% by volume; more preferably from 25% to 35% by volume. Typically, the benzene content in the gasoline-based fuel is at most 5% by volume; preferably at most 2% by volume; more preferably at most 1% by volume. In one embodiment, the benzene content in the gasoline-based fuel is in the range of from 0.1% to 1% by volume based on the volume of the gasoline-based fuel. Preferably, the gasoline base fuel comprises 30% to 90% by volume of saturated hydrocarbons, 2% to 30% by volume of olefinic hydrocarbons, and 15% to 60% by volume of aromatic hydrocarbons, based on the volume of the gasoline base fuel. More preferably, the gasoline base fuel comprises 40% to 80% by volume of saturated hydrocarbons, 5% to 20% by volume of olefinic hydrocarbons, and 25% to 35% by volume of aromatic hydrocarbons. The gasoline base fuel may comprise oxygenated hydrocarbons at from 0.1% to 50% by volume relative to the gasoline base fuel; preferably from 5% to 40% by volume; more preferably from 10% to 40% by volume. Suitable oxygenated hydrocarbons include alcohols (preferably alcohols having from 1 to 4 carbon atoms) and ethers. PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ (preferably ethers having 5 or 6 carbon atoms) such as, for example, methanol, ethanol, isopropyl alcohol, isobutyl alcohol, tert-butyl alcohol, methyl tert-butyl ether, diisopropyl ether, ethyl tert-butyl ether, tert-amyl methyl ether, and the like. Typically, the maximum oxygen content of the gasoline-based fuel is up to 10% by weight based on the weight of the gasoline-based fuel; preferably up to 5% by weight; more preferably up to 3% by weight. In one embodiment, the oxygen content of the gasoline-based fuel is up to 3.7% or up to 2.7% by weight. The gasoline base fuel and gasoline fuel composition preferably have a low or ultra-low sulfur content, for example, at most 0.01% by weight of sulfur based on the weight of the gasoline base fuel or gasoline fuel composition, respectively; preferably at most 0.005% by weight; more preferably at most 0.001% by weight. The gasoline base fuel, and the gasoline fuel composition, preferably has a low lead content, such as at most 5 mg / l, more preferably the gasoline base fuel is lead-free, i.e. no lead compound has been added to it. Gasoline base fuel may be derived from straight-run gasoline, polymeric gasoline, natural gasoline dimer and trimerized olefins, synthetically produced aromatic hydrocarbon mixtures, or from catalytic cracked or thermal cracked petroleum reserves, and mixtures thereof. Preferably, the gasoline base fuel has a research octane rating (RON) greater than 80, such as greater than 85, preferably greater than 90, more preferably greater than 95. The RON is determined according to the method of ASTM D2699. In a preferred embodiment, the gasoline base fuel has a RON greater than 90. In a further preferred embodiment, the gasoline base fuel has a RON greater than 95. In one embodiment, the gasoline base fuel has a RON of from 80 to 110, preferably from 90 to 105, more preferably from 90 to 100. In another embodiment, the gasoline base fuel has a RON of from 85 to 110, preferably from 95 to 105, more preferably from 95 to 100. PQQfr ΙΠ / ΖΖηΖ / Β / ΥΙΛΙ The gasoline fuel composition may further comprise conventional fuel additives, in addition to the anti-knock fuel additive of the invention, such as antioxidants, corrosion inhibitors, detergents, dehazers, metal deactivators, valve seat recession protecting compounds, solvents, carrier fluids, diluents, friction modifiers, dyes and markers. Preferably, the total amount of such conventional fuel additives in the gasoline fuel composition is up to 1% by weight based on the weight of the gasoline fuel composition. In one embodiment, the amount of such conventional fuel additives in the gasoline fuel composition is in the range of from 0.001% to 1.0% by weight based on the weight of the gasoline fuel composition; preferably from 0.01% to 0.8% by weight. In a particular embodiment, the amount of conventional additives is in the range of from 0.01% to 0.4% by weight based on the weight of the gasoline fuel composition. In a particular embodiment, the gasoline fuel composition comprises a multifunctional package comprising one or more of said conventional additives. Suitable antioxidants are known in the art and include phenolic and phenylenediamine compounds, such as 2,4-di-tert-butylphenol, 3,5-di-tert-butyl-4-hydroxy-phenylpropionic acid, BHT, BHB or N,N'-di-sec-butyl-p-phenylenediamine, hydrazine-based antioxidants or thiourea-based antioxidant. The inventors have observed that the use of thiourea-based antioxidants and hydrazine-based antioxidants provides very stable gasoline fuel compositions. Thus, in a particular embodiment, the gasoline fuel composition of the invention comprises a thiourea-based antioxidant, a hydrazine-based antioxidant, or a mixture thereof. In one embodiment, the gasoline fuel composition comprises an antioxidant in an amount of from 2 to 100 mg, preferably from 3 to 30 mg, and more preferably from 4 to 10 mg, per kg of the gasoline fuel composition. Suitable corrosion inhibitors are known in the art. They typically consist of a polar head to allow adhesion to the metal surfaces to be protected, and a hydrocarbon tail to ensure solubility in the fuel. Examples of corrosion inhibitors include carboxylic acids, anhydrides, amines, and amine salts of carboxylic acids. In one embodiment, the gasoline fuel composition may comprise a corrosion inhibitor in an amount of up to 200 mg, preferably up to 100 mg, more preferably up to 50 mg, per kg of the gasoline fuel composition. PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ Suitable detergents (also called deposit control additives) are known in the art. They typically consist of a polar head, the polarity of which is derived from oxygen or nitrogen molecules, and a hydrocarbon tail, which allows the additive to be soluble in the fuel. Examples of detergents include amides, amines, polybutylene succinimides, polyisobutylene amines, polyether amines, polyolefinic amines, and Mannich amines. Preferably, the detergent is a nitrogen-containing or oxygen-containing detergent having a hydrophobic hydrocarbon radical with a number-average molecular weight in the range of 300 to 5000 Da. In one embodiment, the gasoline fuel composition may comprise a detergent in an amount of up to 5000 mg, preferably up to 2000 mg, more preferably up to 1000 mg, per kg of the gasoline fuel composition. For example, in an amount of from 50 to 1000 mg / kg. Suitable mist eliminators (also called demulsifiers or emulsion preventers) are known in the art. They are typically mixtures of alkoxylate compounds including phenolic resins, esters, polyamines, sulfonates or alcohols that have been reacted with ethylene or propylene oxide. Examples of mist eliminators include glycol oxyalkylate mixtures, alkoxylated phenol-formaldehyde polymers, phenol / formaldehyde resin or C1-C1 alkylphenol / formaldehyde oxyalkylates modified by oxyalkylation with C1-C18 epoxides and diepoxides, and C1-C4 epoxide copolymers crosslinked with diepoxides, diacids, diesters, diols, diacrylates, dimethacrylates or diisocyanates, and mixtures thereof.In one embodiment, the gasoline fuel composition may comprise a corrosion inhibitor in an amount of up to 50 mg, preferably up to 10 mg, more preferably up to 5 mg, per kg of the gasoline fuel composition. For example, in an amount of from 0.5 to 5 mg / kg. Suitable solvents, carriers and diluents are known in the art and include synthetic and mineral oils, and solvents. Examples of suitable mineral carrier oils include fractions obtained in the processing of crude oil, such as brightstock or base oils having viscosities, for example, in the SN class 500 - 2000; and also aromatic hydrocarbons, paraffinic hydrocarbons, and alkoxyalkanols. Also useful as a mineral carrier oil is a fraction obtained in the refining of mineral oil and known as hydrocracking oil (a vacuum distillate cut having a boiling range of from about 360 to 500°C, which can be obtained from natural mineral oil that has been subjected to high-pressure catalytic hydrogenation and isomerization and also dewaxing). Examples Suitable synthetic carrier oils PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ are polyolefins (poly-alpha-olefins or poly(internal olefins)), (poly)esters, (poly)alkoxylates, polyethers, polyglycols, aliphatic polyether amines, alkylphenol-initiated polyethers, alkylphenol-initiated polyether amines and carboxylic esters of long-chain alkanols. Any solvent and optionally co-solvent suitable for use in fuels may be used in the gasoline fuel composition of the invention. Examples of suitable solvents for use in fuels include: non-polar hydrocarbon solvents such as kerosene, heavy aromatic solvent, toluene, xylene, paraffins, petroleum, turpentines, and the like. Examples of suitable co-solvents include: polar solvents such as esters and, in particular, alcohols (e.g., t-butanol, ibutanol, hexanol, 2-ethylhexanol, 2-propylheptanol, decanol, isotridecanol, butyl glycols) and mixtures of alcohols. In one embodiment, the gasoline fuel composition may comprise a solvent, carrier or diluent in an amount of up to 8000 mg, preferably up to 5000 mg, more preferably up to 3000 mg, per kg of the gasoline fuel composition. Suitable valve seat recession-protecting compounds are known in the art and include, for example, sodium or potassium salts of polymeric organic acids. In one embodiment, the gasoline fuel composition may comprise a valve seat recession-protecting compound in an amount of up to 200 mg, preferably up to 100 mg, more preferably up to 50 mg, per kg of the gasoline fuel composition. Suitable gasoline friction modifiers are known in the art and include, for example, linoleic acid and its derivatives. In one embodiment, the gasoline fuel composition may comprise a friction modifier in an amount of up to 1000 mg, preferably up to 500 mg, more preferably up to 200 mg, per kg of the gasoline fuel composition. One or more conventional fuel additives, or a multifunctional package comprising the same, may be added to the gasoline base fuel or to a gasoline fuel composition comprising a gasoline base fuel and the anti-knock fuel additive of the invention. In another embodiment, one or more conventional fuel additives, or a multifunctional package comprising the same, may be added to the anti-knock fuel additive of the PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ invention. Therefore, the fuel additive of the invention may comprise one or more conventional fuel additives as described herein. In this case, the total amount of such conventional fuel additives in the fuel additive of the invention is up to 90% by weight based on the total weight of the fuel additive, preferably up to 35% by weight, more preferably up to 15%. In one embodiment, the gasoline fuel composition is free of metallic octane boosters, such as manganese or ferrocene octane boosters. In a further embodiment, the gasoline fuel composition of the invention is free of metallic compounds, i.e., the total content of metallic compounds is less than 0.0005% by weight based on the gasoline fuel composition. The gasoline fuel composition of the invention is suitable for use in an internal combustion engine of the spark ignition type known in the art, such as in automobile engines, as well as in other types of engines, such as off-road and aircraft engines. In a further aspect, the invention is directed to the use of the fuel additive of the invention to improve the octane rating of a gasoline. In another aspect, the invention relates to a method for improving the octane rating of a gasoline, said method comprising adding the fuel additive of the invention to the gasoline. Suitable and preferred embodiments of the fuel additive are as defined herein. As mentioned above, the fuel additive of the invention provides a synergistic improvement in the octane rating of gasoline. The gasoline whose octane rating can be improved or increased with the fuel additive of the invention can be a gasoline fuel base as defined herein or a gasoline fuel composition comprising a gasoline fuel base and one or more conventional fuel additives as defined herein. Suitable and preferred embodiments for the gasoline fuel base, gasoline fuel composition, and conventional fuel additives are as defined hereinabove. The increase in the octane rating of gasoline may be, for example, 1 unit or more, preferably 2 units or more, compared to the octane rating of gasoline. PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ without the fuel additive of the invention. The increase in the octane number of the gasoline fuel composition may be at most 10 units, or at most 5 units, compared to the octane number of the gasoline base fuel. The step of adding the fuel additive to gasoline can be performed at any stage of the fuel supply process, from the refinery to the retail gas station (refueling station), including intermediate storage terminals and transportation devices. Due to the low toxicity of the fuel additive of the invention, it can be advantageously added to gasoline at the retail gas station. The anti-knock fuel additive may be added to a gasoline fuel composition comprising one or more conventional fuel additives. Alternatively, the anti-knock fuel additive may be added to a gasoline base fuel, then one or more conventional fuel additives may optionally be added to the resulting composition. In another embodiment, the anti-knock fuel additive may be blended with one or more conventional additives to provide a mixture that is then added to a gasoline base fuel or gasoline base composition. Examples The invention is illustrated by the following examples which in no way limit the scope of the invention. Example 1: Preparation of gasoline composition comprising an antiknock mixture of m-toluidine (m-tol) and n-methyl-p-anisidine (nmpa) The base gasoline fuel used in the test was an EN-228 automotive gasoline, Premium E5 type (oxygen content less than 2.7%) and with a RON greater than 98. This gasoline includes 900 ml / m3 of Repsol's CTR-BP-6-2827 multifunctional additive package that includes several components, such as: detergent, dispersant, marker, colorant, demulsifier and anti-corrosion additive. NMPA (minimum purity of 95%) and m-toluidine (purity of 98.5%) were mixed at room temperature in equal volume ratios. The resulting mixture was added to the previous gasoline at 1% v / v in sealed flasks to prevent the loss of volatile compounds. The composition and physical properties of the gasoline composition comprising the antiknock mixture are shown in Table I. The data were measured using the test method indicated in the table. PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ Table I Property Test Method Unit is Gasoline Basis Gasoline with 1% v / v of a 50:50 (by volume) blend of m-tol:NMPA Research Octane Number (RON) ASTM D 2699-18a - 98.6 100.8 Motor Octane Number (MON) ASTM D 2700-18a - 87.0 88.4 Density (at 152C) ASTM D 4052-18 Kg / m3 746.7 751.1 Vapor Pressure (DVPE) Winter (01 / 10-10 / 04) Summer (01 / 05-30 / 09) ASTM D 5191-15 KPa KPa 56.4 55.5 Initial Boiling Point ASTM D 86-17 2C 34.3 34.7 % evaporated at 70sC (E70) winter (01 / 10-10 / 04) summer (01 / 05-30 / 09) % v / v % v / v 30 29.5 % evaporated at 1002C (E100) % v / v 55.9 55.3 % evaporated at 1502C (E150) % v / v 85.8 84.6 Final boiling point 2C 203.5 208.0 Distillation residue % v / v 1 1.1 Distillation loss % v / v 0.3 0.9 VLI (10 VP + 7 E70) - 774 762 Sulfur content ASTM D 4294-16e1 mg / Kg 7.6 6.8 Lead content UNE EN 237:2005 mg / L <0.0025 <0.0025 Copper strip corrosion (3h at 502C) ASTM D 130-18 classification 1A 1A Oxidation stability ASTM D 525-12a min >960 >960 Existing gum content (washed) ASTM D 381-12 (2017) mg / 100 mL <0.5 <0.5 Hydrocarbons: defins Aromatics UNE EN ISO 22854:2016 % v / v % v / v 10.8 29.8 10.9 30. why? ιη / ζζηζ / Β / γΐΛΐ Saturates Benzene % v / v % v / v 45.8 0.62 45.1 0.63 Oxygen content % m / m 2.34 2.4 Oxygenate content methanol % v / v 0.15 0.16 ethanol % v / v 0.65 0.66 isopropyl alcohol % v / v <0.01 <0.01 tert-butyl alcohol % v / v <0.01 0.09 isobutyl alcohol % v / v <0.01 <0.01 MTBE % v / v 0.36 0.36 ETBE % v / v 12.44 12.7 ethers with 5 or more C % v / v 12.8 13.07 other oxygenates % v / v <0.01 <0.01 PQQfr ίΠ / ΖΖηΖ / Β / ΥΙΛΙ As shown in Table I, the anti-knock additive increased the octane rating of gasoline while maintaining the specifications of EN-228 automotive gasoline. Example 2: Determination of synergy for the antiknock mixture of m-toluidine (mtol) and N-methyl-p-anisidine (NMPA) in Premium gasoline (RON > 98) The gasoline base fuel used in the test was an EN-228 automotive gasoline as described in Example 1. NMPA (minimum purity of 95%) and m-toluidine (98.5% purity) were used as octane improvers. They were added to gasoline in the indicated amounts in sealed flasks to prevent the loss of volatile compounds, and mechanical magnetic stirring was applied at room temperature until complete dissolution was observed. In the case of a mixture of anilines, they were premixed at room temperature in the indicated volume ratio. Research octane number (RON) measurements were carried out according to the ASTM D 2699-18a method. Motor octane number (MON) measurements were carried out according to the ASTM D2700-18a method. Engine antiknock requirements are associated with a combination of RON and MON. The octane number used in this document is based on the most recent developments in CONCAWE for modern engines, where octane number = 1.6*RON - 0.6*MON, where high motor octane numbers lead to poorer antiknock performance. Predicted RON values ​​were calculated for mixtures of m-toluidine and NMPA at different volume ratios by linear volume mixing. The uncertainty of the results was estimated at 95% confidence level as 2*Sr. Sr2 is the statistical estimator of the variance under repeatability conditions for the A / C bounding procedure of ASTM D2699. Given a repeatability of 0.2, the estimated uncertainty was 0.14. Some control samples were introduced throughout the experimental program: see entries 5, 9 and 14. The differences between 0.0 and 0.1 with respect to the previous results (entries 4, 8 and 13) allowed us to validate the estimated uncertainty. The results obtained are shown in Table II. pool? tn / zznz / e / YiAi Table II Additive input (% v / v) Additive dosage (% v / v) RON ASTM D2699 theoretical RON MON ASTM D2700 octane number theoretical octane number 1 - - 98.5 - 87.2 105.20 - 2 m-toluidine 0.25 98.8 - 87.9 105.34 - 3 0.50 99.3 - 88.1 106.02 - 4 1.00 100.0 100.05 88.6 106.94 106.92 5 100.1 88.7 106.89 6 2.00 101.5 - 90.3 108.22 - 7 75% m-tol 25% NMPA 1.00 100.3 100.35 88.4 107.44 107.47 8 50% m-tol 50% NMPA 1.00 100.8 100.65 88.4 88.5 108.24 108.03 9 100.8 108.18 10 25% m-tol 75% NMPA 1.00 101.2 100.95 88.3 108.94 108.58 11 NMPA 0.25 99.4 - 87.5 106.54 - 12 0.50 100.6 - 87.8 108.28 - 13 1.00 101.2 101.25 88.2 109.00 109.14 14 101.3 88.0 109.28 The experimental RON results for these mixtures are shown in Figure 1 (dotted line), compared to the predicted RON for these mixtures (dashed line). The uncertainty level of the experimental results for each point is indicated for each result. As seen in the figure, N-methyl-p-anisidine and m-toluidine behave synergistically at the claimed volume ratios; the observed RON values ​​were higher than those expected for these mixtures. This synergistic behavior for RON is very important in the engine due to its impact on the octane rating. Figure 2 compares the octane rating versus RON values ​​for different blends of NMPA and m-toluidine (dotted lines). Blends with approximately 50% NMPA in m-toluidine or more result in higher octane ratings than expected (dashed line). This means better antiknock performance in the engine for a given RON value. The actual performance of the 75% NMPA / 25% m-toluidine blend is close to that of 100% NMPA, at a lower cost due to the lower cost of m-toluidine. Example 3: Determination of synergy for the antiknock mixture of m-toluidine (mtol) and N-methyl-p-anisidine (NMPA) in low octane gasoline (RON > 91) with isopropyl alcohol The gasoline base fuel used in the test was a commercial EN-228 conventional gasoline blend (RON 95), which was blended with 5% v / v of conventional low-octane refining component at room temperature in a sealed flask. This blend meets all EN-228 specifications except for anti-knock properties, which are designed to meet only the regular-grade octane requirements of RON 91 of the 6eWorld Wide Fuel Charter Category 2 and 3 gasoline specifications (March 2019). The anti-knock properties of the final blend were RON 93.2 (ASTM D2699) and MON 83.5 (ASTM D2700). The experimental tests for octane number as well as the criteria for theoretical RON and uncertainty were the same as in Example 2. The NMPA and m-toluidine materials and the mixing procedure were the same as in Example 2. Isopropyl alcohol (99.8% purity) was mixed with the above gasoline in the indicated amount in closed flasks in order to prevent the loss of volatile compounds. The results obtained are shown in Table III. why? Ln / zznz / e / YiAi Table III Input m-Toluidine (% v / v) NMPA (% v / v) Isopropyl alcohol (% v / v) RON ASTM D2699 Theoretical RON 1 1.00 0.00 0.50 95.8 95.80 2 0.50 0.50 0.50 96.6 96.45 3 0.25 0.75 0.50 97.0 96.78 4 0.00 1.00 0.50 97.1 97.10 The experimental RON results for these blends are shown in Figure 3 (dotted line), compared to the predicted RON for these blends (dashed line). The uncertainty in the experimental results for each point is indicated by the ascending / descending solid lines. As seen in the figure, blends of N-methylp-anisidine and m-toluidine at the claimed volume ratios also resulted in a synergistic effect in a low-octane gasoline. The observed RON values ​​were higher than the expected values ​​for these blends. Example 4: Cloud point of NMPA mixtures with various solvents Cold flow performance was evaluated by two methods: cloud point determination according to ASTM D7689 (Example 4A) and storage at different temperatures (Example 4B). Example 4A The conventional method according to ASTM D7689 was used as the screening method to evaluate low-temperature performance. Internal testing has shown that samples diluted in NMPA that perform well at low temperatures (-10°C) have cloud point values ​​below -55°C in this test. NMPA and m-toluidine were the same products as those used in Example 2. The ethanol was a commercial sample meeting the requirements of EN-15376. Ethyl t-butyl ether (ETBE) and ethylbenzene were industrial process samples from a Spanish Repsol refinery. 2-Butoxyethanol was purchased from Sigma Aldrich. They were mixed with NMPA in closed flasks to prevent the loss of volatile compounds, and magnetic mechanical stirring was applied at 40 sC until complete dissolution was observed. Low-temperature tests were carried out without delay after mixing to ensure no memory effect and the same starting temperature for all tests. The results obtained are shown in Table IV. why? tn / zznz / e / YiAi Table IV Entrada NMPA (% v / v) Disolvent used Disolvent (% v / v) Punto de enturbiamiento ASTM D7689 1 10 ETBE 90 < -105 2 20 80 -17 3 50 50 -18 4 70 30 -22 5 10 Etilbenceno 90 < -105 6 20 80 -64 7 50 50 -48 8 70 30 -25 9 90 10 -7 10 10 Etanol 90 < -105 11 20 80 -55 12 50 50 -22 13 70 30 -22 14 90 10 -4 15 10 2-butoxyetanol 90 < -105 16 20 80 < -105 17 50 50 -81 18 70 30 -30 19 10 m-toluidina 90 < -105 20 20 80 < -105 21 50 50 -95 22 70 30 -64 23 90 10 -10 24 100 Sin solvent 0 -1 pqqi? tn / zznz / e / YiAi The results are summarized in Figure 4. As shown in this figure, m-toluidine was found to be the best solvent for NMPA, preventing its crystallization at low temperatures even at much lower m-toluidine concentrations than the other solvents tested. This further supports the use of m-toluidine in combination with NMPA. Not only does it enhance the RON enhancement provided by NMPA, but it also improves NMPA handling even when it is present as the majority component in the mixture. Example 4B Storage at different temperatures is the best method to ensure the performance of an additive. After homogenizing 30 grams of the sample for 10 minutes at 40°C, it was stored in a tightly sealed 50 ml container at different temperatures. The sample was stored for one month at 0°C and -10°C. The deposits and appearance of different phases were periodically monitored; the results after 4 weeks are shown in this example. The solvents and mixing procedures were the same as described in Example 4A. The results obtained are shown in Table VI. Table VI NMPA Input (% v / v) Solvent used Solvent (% v / v) Storage temperature (2C) Deposits and appearance after 4 weeks 1 10 ETBE 90 -10 No deposits, clear and shiny 2 20 80 Some deposits 3 50 50 Some deposits 4 70 30 100% solid 6 10 90 0 No deposits, clear and shiny 7 20 80 Some deposits 8 50 50 100% solid 9 70 30 100% solid 5 10 15 20 25 30 35 10 10 Ethylbenzene 90 -10 No deposits, clear and bright 11 20 80 No deposits, clear and bright 12 50 50 Some deposits 13 70 30 100% solid 14 10 90 0 No deposits, clear and bright 15 20 80 No deposits, clear and bright 16 50 50 Some deposits 17 70 30 100% solid 18 10 Ethanol 90 -10 No deposits, clear and bright 19 20 80 No deposits, clear and bright 20 50 50 Some deposits 21 70 30 Some deposits 22 10 90 0 No deposits, clear and bright 23 20 80 No deposits, clear and bright 24 50 50 100% solid 25 70 30 Some deposits 26 10 2-butoxyethanol 90 -10 No deposits, clear and glossy 27 20 80 No deposits, clear and glossy 28 50 50 No deposits, clear and glossy 29 70 30 No deposits, clear and glossy 30 10 90 0 No deposits, clear and glossy 31 20 80 No deposits, clear and glossy 32 50 50 No deposits,clear and glossy 33 70 30 No deposits, clear and glossy 34 20 m-toluidine 80 -10 No deposits, clear and glossy 35 50 50 No deposits, clear and glossy 36 70 30 No deposits, clear and glossy 37 20 80 0 No deposits, clear and glossy 38 50 50 No deposits, clear and glossy 39 70 30 No deposits, clear and glossy 40 100 0 -10 100% solid, pqqi? Ln / zznz / e / γΐΛΐ 41 100 Solvent-free 0 0 100% solid pqqi? Ln / zznz / e / γΐΛΐ The discrete screening selection (30, 50, 80, and 90% solvent) provided, in most cases, similar behavior at -10 sC and 0 sC; the results were most consistent at -10 sC. The worst performance was with ETBE mixtures, where only samples with a 90% dilution showed no deposits and maintained a clear, glossy appearance at the end of the tests. In the cases of aromatic compounds (ethylbenzene) or alcohol (ethanol), the minimum solvent requirement was 80%. The best performance was observed for 2-butoxyethanol and m-toluidine, where 30% solvent was sufficient to ensure appropriate performance even at -10 sC. These results further demonstrate the ability of m-toluidine to act as a solvent for NMPA. Example 5: Determination of the compatibility of the antiknock mixture of m-toluidine (m-tol) and N-methyl-p-anisidine (NMPA) with engine lubricating oil The test used to evaluate compatibility with engine lubricant was a modification of the CEC L-109-14 oxidation test for engine lubricants, in which lubricating oil was mixed with 4.5% v / v gasoline to simulate the diluted lubricating oil found in engines. The engine lubricant to be tested was also contaminated with aniline or the aniline mixture used as an octane booster. The concentration of gasoline in the lubricating oil tested is considered representative of fuel contamination in an aged engine lubricant in typical use in light-duty vehicles, and this level of contamination is widely accepted in lubricant testing. Aniline contamination in the lubricant was evaluated in the range of 0.5 to 1.5% by volume. The selected contamination amount range to simulate aging conditions in a real engine lubricant was based on data from an experimental engine endurance test using a gasoline with an aniline octane booster. The fuel used in the engine endurance test was a commercial premium gasoline (RON 98, 900 ml / m3 multifunctional additive package) with sufficient NMPA-based octane booster to increase the RON above 100. The engine in this test was an M271 E18. The engine was operated for over 270 hours under very severe conditions to promote sludge formation in the lubricating system.Following this endurance test, the engine lubricant was analyzed for aniline and fuel contamination: the aniline concentration was less than 0.6% w / w and the fuel dilution was less than 7% w / w. Therefore, the selected range for aniline contamination in the engine lubricant tested on a laboratory scale, from 0.5 to 1.5%, was considered representative and sufficiently rigorous. The final mixture was stored at 1562°C for 168 hours, using 100 mg / kg of Fe (Fe-III acetylacetonate) as a catalyst and an air flow of 10 L / h. Engine oil oxidation was monitored throughout the test using various parameters, including the kinematic viscosity of the oil at 100°C, measured according to ASTM D445. Observation of remaining deposits in the empty flask after the test is a very useful method for monitoring oil stability. Two engine lubricants were used in these examples: Repsol Elite Evolution Long Life 5W30, a common low-sulfated ash lubricant for light-duty engines that meets ACEA A3 specifications; and Repsol Elite Common Rail 5W30, a high-sulfated ash lubricant that meets ACEA A3 / B4 specifications. The gasoline base stock, NMPA, and m-toluidine used were the same as in Example 1. The lubricant, gasoline and anilines were mixed in closed flasks to prevent the loss of volatile compounds, and magnetic mechanical stirring was applied at room temperature until complete dissolution was observed. The results obtained are shown in Table VII. why? Ln / zznz / e / YiAi Table VII Engine oil Engine oil (% v / v) Gasoline contamination (% v / v) Octane booster contamination (% v / v) Octane booster blend Viscosity at start of test (mm2 / s at 100sC) Viscosity at end of test (mm2 / s at 100aC) Repsol Elite Evolution Long Life 5W30 - ACEA C3 (low sulphated ash) 95.5 4.5 0.0 — 9.651 8.901 99.5 0.5 NMPA 100% 9.558 8.137 99.0 1.0 9.393 8.140 98.5 1.5 9.233 9.661 99.5 0.5 50% v / v NMPA and mtol 10.330 9,347 99.0 1.0 10,120 9,580 98.5 1.5 9,730 9,777 Repsol Elite Common Rail 5W30 - ACEA A3 / B4 (high 100.0 0.0 — 9,346 10,330 98.5 1.5 NMPA 100% 8,899 10,900 sulfated ash) 99.0 1.0 50% v / v NMPA and m-tol 9.367 10.710 pqqi? Ln / zznz / e / γΐΛΐ The observation of adherent insoluble products and viscosity change in contaminated engine lubricant demonstrate that mixing m-toluidine with NMPA improves compatibility with the engine lubricant. Viscosity loss is associated with cracking degradation, while polymerization degradation increases viscosity. As shown in the table above, the mixture of NMPA and m-toluidine, for the same aniline concentration, provided a lower viscosity change compared to the conventional reference used oil at any contamination level. The drained flask after the aging test of the ACEA C3 engine lubricant is shown in Figure 5. Adherent insoluble degradation products remain in the flask. The adhering insoluble products were reduced with m-toluidine, and this improvement increased as the amount of octane booster increased, clearly observed at 1.5% aniline contamination. The same trend was observed when testing the compatibility of different octane booster blends with ACEA A3 / B4 (high sulphated ash) engine lubricant.

Claims

1. Fuel additive comprising a mixture of N-methyl-p-anisidine and m-toluidine in a volume ratio from 45:55 to 95:

5.

2. Fuel additive according to claim 1, wherein the volume ratio of N-methyl-p-anisidine to m-toluidine is from 45:55 to 90:10, preferably from 45:55 to 85:

15.

3. Fuel additive according to claim 2, wherein the volume ratio of N-methyl-p-anisidine to m-toluidine is from 50:50 to 90:10, preferably from 50:50 to 75:

25.

4. Fuel additive according to any one of claims 1 to 5, further comprising an antioxidant.

5. Gasoline fuel composition comprising a fuel additive as defined in any one of claims 1 to 4 and a gasoline base fuel.

6. Gasoline fuel composition according to claim 5, comprising from 0.2% to 5% by volume of the fuel additive in relation to the total volume of the gasoline fuel composition.

7. Gasoline fuel composition according to claim 6, comprising from 0.5% to 3% by volume of the fuel additive in relation to the total volume of the gasoline fuel composition.

8. Gasoline fuel composition according to claim 5, comprising from 0.2% to 5% by volume relative to the volume of the gasoline fuel composition, preferably from 0.5% to 3% by volume, of a combination of m-toluidine and NMPA, wherein the volume ratio of NMPA to m-toluidine is from 45:55 to 95:5, preferably from 45:55 to 90:

10.

9. Gasoline fuel composition according to claim 8, wherein the volume ratio of NMPA to m-toluidine is from 45:55 to 85:15, preferably from 50:50 to 75:

25.

10. Gasoline fuel composition according to any one of claims 5 to 9, wherein the gasoline base fuel comprises saturated hydrocarbons, olefinic hydrocarbons, aromatic hydrocarbons and, optionally, oxygenated hydrocarbons.

11. Gasoline fuel composition according to claim 10, comprising from 30% to 90% by volume of saturated hydrocarbons, from 2% to 30% by volume of olefinic hydrocarbons, and from 15% to 60% by volume of aromatic hydrocarbons and, optionally, from 0.1% to 50% of oxygenated hydrocarbons.

12. Gasoline fuel composition according to any one of claims 5 to 11, wherein the gasoline composition further comprises one or more additives selected from the group consisting of antioxidants, corrosion inhibitors, detergents, mist eliminators, metal deactivators, valve seat recession protectants, colorants, solvents, carrier fluids, friction modifiers, thinners, and markers.

13. Gasoline fuel composition according to any one of claims 3 to 12, wherein the gasoline base fuel has an octane rating of from 80 to 110, preferably from 90 to 100, as determined according to ASTM D2699.

14. Use of a fuel additive as defined in any one of claims 1 to 4 to improve the research octane rating of a gasoline.

15. Method for improving the octane rating of a gasoline, comprising adding the fuel additive according to any one of claims 1 to 4 to the gasoline.