Methanol-based engine fuel containing a combustion-enhancing additive

Abstract

The invention relates to a fuel for a compression ignition engine, comprising 98.0% to 99.9% by mass of methanol and 0.01% to 2.0% by mass of an alkyl nitrate or a mixture of alkyl nitrates.

Description

[0001] Methanol-based engine fuel containing a combustion-enhancing additive.

[0002] Field of invention

[0003] The field of the invention is that of fuels for combustion engines. The fuel of the invention is one of the new fuels with reduced environmental impact, for example those commonly called "e-fuels" when they are produced from low-carbon electricity, low-carbon hydrogen, and / or from CO2. They are considered as a solution for the decarbonization of transport. The fuel of the invention is essentially (at least 98%) composed of methanol and is thus one of the ecological alternatives for replacing fossil fuels. The invention relates to an additive which, incorporated into methanol, ensures better ignition and faster combustion of the fuel in the engine.

[0004] State of the art

[0005] Methanol, such as "e-fuels," represents a credible low-impact alternative to replace fossil fuels and biofuels in combustion engines in the near future. Combustion of the resulting methanol results in a CO2-neutral balance. Methanol as a fuel has a usable energy density in combustion engines but has a low cetane number. Its ignition in compression-ignition engines, such as diesel, is problematic, especially at low engine speeds. Various methods for improving the ignition of methanol in a compression-ignition engine have been described.

[0006] The first method is the co-injection of a pilot fuel or an additive into the engine with methanol. For example, patent application CN 214944586 describes the co-injection of a diesel-type pilot fuel with methanol as the main fuel. The scientific article "Effect of Cetane Improvers on Gasoline, Ethanol, and Methanol Reactivity and the Implications for RCCI Combustion" (SAE International Journal of Fuels and Lubricants Vol. 6, No. 1 (April 2013), pp. 170-187) describes the co-injection of 2-ethylhexyl nitrate (NEH) with a fuel composed primarily of methanol. During combustion in a specific RCCI (Reactivity Compression Controlled Ignition) mode, a well-mixed low-reactivity fuel and oxidizer (usually air) are compressed but do not achieve autoignition.Later, still during the compression cycle, high reactivity fuel, here NEH, is injected to form a local mixture of low and high reactivity fuel. The mass rates of co-injected NEH are approximately 6%. This type of process has the major disadvantage of requiring co-injection control according to the engine operating conditions, structural arrangements such as separate tanks and injection systems. Also, the use of a fuel as a pilot fuel generates CO2 emissions with a negative impact on the environment.

[0007] The article "Effect of cetane Improvers on Gasoline, Ethanol and Methanol Reactivity and the Implications for RCCI Combustion" (SAE International Journal of Fuels and Lubricants, Vol. 6, No. 1, 2013-04-08, pages 170-187) concerns a study of the combustion of gasoline, ethanol and methanol fuels in a compression ignition engine in the presence of an injected additive, DTPB or NEH. The combustion efficiency of fuels with additives is studied in comparison with that of a reference fuel (PFR = Primary Reference Fuel) consisting of a mixture of isooctane and n-heptane. The combustion results of fuels with additives are presented as the equivalent of the efficiency of the PRF number (PRF number) as a function of the mass percentage of additive in the fuel. The tests are carried out at high inlet temperatures (Tin) of 113°C and 82°C for which the methanol fuel is gaseous.These high intake temperatures are necessary to ensure the good mixing of the injected NEH additive with methanol with additive mass rates between ~1% and 18%. This article shows that the injection of NEH in the presence of gaseous methanol fuel allows for a high mass rate of approximately 7% to achieve an acceptable combustion efficiency (Effective PRF# = 70) in reference to the standardized fuel. It is concluded from this article that a mass rate of NEH of 7% injected in the presence of gaseous methanol is likely to lead to an acceptable combustion efficiency.

[0008] The second method is a mixture of fuels with methanol. It is known to mix alcohols, such as methanol or ethanol, in minority or majority proportions with fossil or synthetic diesel fuels such as dimethyl ether (DME) or with gasoline. In this case, the CO2 emissions balance has a negative impact on the environment.

[0009] The third method is the incorporation of an ignition-enhancing additive mixed with methanol, possibly also mixed with diesel fuel. Many patent applications, such as patent applications CN 103865592 and CN 104232180, describe the incorporation of a cocktail of multifunctional additives mixed with methanol, possibly also mixed with fuel. These additives have, for example, preservation, anti-corrosion, combustion-improving, and detergency functions. These additives generally represent at least 10% of the mass of the fuel. The proportion of methanol in the fuel is between 30% and 90% by mass depending on the mass percentages of additives and diesel fuel present in the fuel. The combined effects of these additives are only observed globally without precisely measuring the necessary quantities. It is therefore not a fuel essentially composed of methanol.

[0010] On another level, in the field of hydrocarbon diesel or biodiesel fuels, 2-ethylhexyl nitrate (NEH) has been known for a long time as a cetane improver for diesel fuel. A higher cetane value ensures lower fuel consumption, reduced particulate matter and NOx emissions, faster cold engine starting, reduced engine knocking and noise, and reduced engine wear. The reaction mechanism of NEH in the presence of a hydrocarbon diesel fuel has been studied, for example, in the scientific publication "The Autoignition Behavior of Surrogate Diesel Fuel Mixtures and the Chemical Effects of 2-Ethylhexyl Nitrate (2-EHN) Cetane Improver" (vol. 108, section 4: Journal of fuels and lubricants (1999), pp. 1029-1045).However, the reaction mechanism of its effect on diesel fuel cetane improvement remains poorly understood and its use is based on empirical laws. For this reason, its effectiveness on fuels other than diesel fuels cannot be assumed. This additive is industrially produced and widely used in commercial diesel fuels. More than fifty thousand tons of NEH have been produced annually in Europe since the 1980s. Other alkyl nitrates can also be used as diesel fuel cetane improvement additives. Figure 1 shows the similarity to NEH of the effects on the cetane number of different alkyl nitrates in addition to a standard diesel fuel (medium paraffinic (~40%) with a low natural cetane number but a standard response to the cetane improver).The cetane index improvement effect is obtained for an alkyl nitrate incorporation in diesel fuel close to 0.03% by mass.

[0011] The present invention provides a solution for improving the ignition of methanol in combustion engines. This improvement is obtained by adding in a very small mass proportion the NEH additive alone in a mixture with the methanol fuel. More broadly, other alkyl nitrates, whose power to improve the cetane index is known in a similar way to NEH for a diesel fuel, are also recommended within the scope of the present invention to improve the ignition of methanol. It is unexpected that additives known to increase the cetane index of a diesel or biodiesel hydrocarbon can be so effectively used, at a very low mass percentage, to improve the ignition of an alcohol such as methanol.

[0012] Summary of the invention

[0013] The invention relates to a fuel comprising from 98.0% to 99.9% by mass of methanol for an engine by compression auto-ignition or by spark ignition, and from 0.01% to 2.0% by mass of a compound for improving the ignition delay of methanol.

[0014] The said compound is an alkyl nitrate.

[0015] The said compound has the advantage of being liquid at room temperature, not very flammable, non-toxic and produced industrially.

[0016] The said compound, at these low levels in the presence of methanol fuel, is therefore conventionally considered a fuel additive.

[0017] The addition of said liquid compound is carried out in a mixture with methanol in the liquid state to form the fuel according to the invention in a tank. It can also be stored separately and mixed with methanol to form the fuel according to the invention prior to its injection into the engine, or co-injected to form the fuel according to the invention in a premixing chamber of the engine.

[0018] The said compound, of the alkyl nitrate type, previously reserved for improving the cetane index of fossil diesel or biodiesel hydrocarbon fuels, is therefore used effectively and surprisingly as additives to improve the ignition of methanol.

[0019] Brief description of the figures

[0020] Figure 1 shows the effect of alkyl nitrates on the cetane number of a standard diesel fuel.

[0021] Figure 2 shows the effect of NEH on the ignition delay of methanol for a richness (air / fuel) of 0.5 at a compression pressure Pc of 30 bars: left curve = fuel consisting of 100% methanol; right curve = fuel consisting of 99.8% methanol and 0.2% NEH.

[0022] Figure 3 shows the effect of NEH on the ignition delay of methanol for a richness (air / fuel) of 1 at a compression pressure Pc of 30 bars: left curve = fuel consisting of 100% methanol; right curve = fuel consisting of 99.8% methanol and 0.2% NEH.

[0023] Figure 4 shows the effect of NEH on the ignition delay of methanol for a richness (air / fuel) of 1.5 at a compression pressure Pc of 30 bars: left curve = fuel consisting of 100% methanol; right curve = fuel consisting of 99.8% methanol and 0.2% NEH.

[0024] Figure 5 shows the ignition delay of a methanol-based fuel containing different amounts of NEH as a function of temperature, for a richness (air / fuel) of 1 at a compression pressure Pc of 30 bars: curves from left to right = fuel consisting of 100% methanol; fuel consisting of 99.8% methanol and 0.2% NEH; fuel consisting of 99.5% methanol and 0.5% NEH; fuel consisting of 99.0% methanol and 1.0% NEH.

[0025] Figure 6 shows the ignition delay of a methanol-based fuel containing different amounts of NEH, for a richness (air / fuel) of 1, at a compression pressure Pc of 30 bars, over a temperature range from 850K to 1000K.

[0026] Figure 7 is an enlarged view of Figure 6 over the temperature range from 900K to 1000K.

[0027] Description of the invention

[0028] The present disclosure relates to an engine fuel (particularly for a combustion engine) which comprises about 98.0% to about 99.9% by mass of methanol and about 0.01% to about 2.0% by mass of a compound consisting of an alkyl nitrate. In one embodiment, the fuel comprises about 0.05% to about 1.5% by mass of said compound. In another embodiment, the fuel comprises about 0.1% to about 1.0% by mass of said compound. In another embodiment, the fuel comprises about 0.1% to less than 1.0% (<1.0%) by mass of said compound.

[0029] In one embodiment, the fuel of the invention consists of methanol and said compound (and in this case, the amount of compound in the fuel is at least 0.1% by mass).

[0030] In another embodiment, when the sum of the amount of methanol and the amount of compound is not equal to 100% by mass, the fuel may contain one or more other additives to make the fuel 100%, such as additives with preservative, anti-corrosion or detergency functions.

[0031] Said compound added to methanol is chosen from one or more linear, branched or cyclic alkyl nitrates.

[0032] Said compound is more particularly chosen from linear alkyl nitrates comprising 4 to 36, advantageously 4 to 24 carbon atoms, branched alkyl nitrates comprising 4 to 36, advantageously 4 to 24 carbon atoms, cyclic alkyl nitrates (or cycloalkyl nitrates) comprising 5 to 18 carbon atoms, and mixtures thereof. In one embodiment, said compound is chosen from 2-ethylhexyl nitrate, cyclohexyl nitrate, dodecyl nitrate, n-nonyl nitrate, 2-tetradecyl-1-octadecyl nitrate, hexyl nitrate, 2-octyl nitrate, isononyl nitrate, 2-propylheptyl nitrate, a mixture of C9-C13 branched alkyl nitrates, and mixtures thereof. In one embodiment, the alkyl nitrate is 2-ethylhexyl nitrate alone or in a mixture with one or more other alkyl nitrates as defined above, advantageously the alkyl nitrate is 2-ethylhexyl nitrate alone.

[0033] Mixtures of C9-C13 branched alkyl nitrates may be obtained from corresponding mixtures of C9-C13 branched alcohols, for example, alcohols available from Exxon under the trade name Exxal™. As a guide, a mixture of at least two branched alcohols selected from a C9 branched alcohol, a C branched alcohol, or a C i0 , a Cn-branched alcohol, a C-branched alcohol i2 and a C-branched alcohol i3 , then synthesize the corresponding alkyl nitrate mixture.

[0034] According to one embodiment, said compound and the methanol are stored separately, and brought into contact with each other in an injector, thus forming the fuel according to the invention before it is conveyed into the combustion chamber of the engine.

[0035] According to one embodiment, said compound is stored separately from the methanol and is co-injected with the methanol to form the fuel according to the invention in a premixing chamber of the engine.

[0036] The present disclosure also relates to the use of an alkyl nitrate (as defined above), in the proportions defined above, as an ignition enhancer for a fuel based on (consisting of) methanol.

[0037] The present disclosure also relates to a method for improving the ignition of a methanol-based (consisting of) fuel in an engine (especially a combustion engine), the method comprising adding to the methanol an alkyl nitrate (as defined above), in the proportions defined above. In one embodiment, the alkyl nitrate and methanol are mixed in an injector. In one embodiment, the alkyl nitrate and methanol are mixed in a premixing chamber of the engine.

[0038] The present disclosure also relates to an engine (in particular a combustion engine) of a vehicle (such as a car, heavy goods vehicle, tractor, etc.) or a ship (such as an oil tanker, container ship, etc.) containing a fuel as defined above. The present disclosure also relates to a vehicle or ship comprising an engine as defined above.

[0039] The invention is illustrated by the following examples, given for information purposes only. Examples

[0040] The improvement in the ignition delay of methanol was measured under test conditions equivalent to those described in the scientific article “Ignition delay times of NH3 / DME blends at high pressure and low DME fraction: RCM experiments and simulations” (Combustion and Flame Volume 227, May 2021, Pages 120-134). The laboratory test engine is a rapid compression machine equivalent to the one described in this scientific article. It is a rapid compression machine for measuring the self-ignition delay of a mixture. This machine allows the mixture to be compressed in a very short time in order to obtain pre-established pressure and temperature conditions. The liquids are admitted into the tank through a different orifice than the gas inlet; the quantities of liquids are measured using a syringe and a precision balance.

[0041] The ignition delay dAI is defined according to the following formula in which Pc is the pressure applied to the injected fuel:

[0042] Example 1

[0043] The ignition delays were determined as a function of the injection temperature (between 800K and 1000K), at a pressure Pc of 30 bars, of a fuel according to the invention consisting of 99.8% by mass of methanol and 0.2% by mass of NEH and for a mixture richness of 0.5, 1 and 1.5 respectively with air. The results are shown in Figures 2, 3 and 4 with the reference points of the ignition delay of methanol alone. A significant reduction, by approximately a factor of 5 to 10, in the ignition delays of the fuel is observed when the latter contains NEH, compared to methanol alone. This reduction in the ignition delay compared to that of methanol alone is all the more significant as the temperature is low.

[0044] Example 2

[0045] The ignition delays of different fuels containing either methanol alone or a mixture of methanol and NEH at a mass ratio of 0.2%, 0.5% or 1% were determined, at different injection temperatures (temperature range from 790K to 1000K), at a pressure Pc of 30 bars and a mixture richness of 1 with air. As can be seen in Figures 5 to 7, the addition of NEH to a fuel consisting of methanol results in a significant reduction in the ignition delay compared to methanol alone. These examples show that the use of alkyl nitrate(s) in a very low mass percentage makes it possible to significantly improve the ignition delay of a methanol-based fuel. Nothing could have suggested that additives known to increase the cetane number of a diesel or biodiesel hydrocarbon could be used as effectively, in very small quantities, to improve the ignition of an alcohol such as methanol.

Claims

Demands 1. Fuel for compression-ignition engines comprising 98.0% to 99.9% by mass of methanol in liquid form and 0.01% to 2.0% by mass of an alkyl nitrate in liquid form.

2. Fuel according to claim 1, comprising 0.05% to 1.5% by mass, in particular 0.1% to 1.0% by mass of said alkyl nitrate.

3. Fuel according to any one of claims 1 and 2, wherein the alkyl nitrate is a linear alkyl nitrate comprising from 4 to 36 carbon atoms, a branched alkyl nitrate comprising from 4 to 36 carbon atoms, a cyclic alkyl nitrate comprising from 5 to 18 carbon atoms, or a mixture of these nitrates.

4. Fuel according to any one of claims 1 to 3, wherein the alkyl nitrate is selected from 2-ethylhexyl nitrate, cyclohexyl nitrate, dodecyl nitrate, n-nonyl nitrate, 2-tetradecyl-1-octadecyl nitrate, hexyl nitrate, 2-octyl nitrate, isononyl nitrate, 2-propylheptyl nitrate, a mixture of C9-C13 branched alkyl nitrates, and mixtures thereof 5. Fuel according to claim 4, wherein the alkyl nitrate is 2-ethylhexyl nitrate.

6. Fuel according to any one of claims 1 to 5, which consists of 98.0% to 99.9% by mass of methanol in liquid form and 0.01% to 2.0% by mass of alkyl nitrate in liquid form.

7. A process for obtaining a fuel according to any one of claims 1 to 6, comprising mixing alkyl nitrate in liquid form with methanol.

8. A method according to claim 7, wherein methanol and alkyl nitrate are mixed in an engine tank.

9. A method according to claim 7, wherein methanol and alkyl nitrate are mixed in an injector.

10. A method according to claim 7, wherein methanol and alkyl nitrate are mixed in a premixing chamber of an engine.

11. Use of an alkyl nitrate in liquid form, at a rate of 0.01% to 2.0% by mass, in a liquid methanol-based fuel, as an ignition enhancer for said fuel.

12. Use according to claim 11, wherein the alkyl nitrate is as defined in any one of claims 3 to 5.

13. Method for improving the ignition, in an engine, of a fuel comprising from 98.0% to 99.9% by mass of liquid methanol, comprising the addition to the methanol of a liquid alkyl nitrate, at a rate of 0.01% to 2.0% by mass, preferably from 0.05% to 1.5% by mass, preferably again from 0.1% to 1.0% by mass.

14. Method according to claim 13, wherein the alkyl nitrate is as defined in any one of claims 3 to 5.

15. Method according to any one of claims 13 and 14, wherein alkyl nitrate and methanol are mixed in an injector.

16. Method according to any one of claims 13 and 14, wherein alkyl nitrate and methanol are mixed in a premixing chamber of the engine.

17. Vehicle or ship engine containing a fuel as defined in any one of claims 1 to 6.

18. Vehicle or vessel comprising an engine as defined in claim 17.

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