Fuel for compression engines

Abstract

A fuel for a combustion engine, comprising 95.00 to 99.75% by mass of ammonia and 0.25 to 5.00% by mass of pilot fuel, wherein the pilot fuel is composed of hydrocarbons with a cetane number exceeding 60 and also contains 0.02 to 30.0% by mass of alkyl nitrate or a mixture of alkyl nitrates. The present invention also relates to a method for obtaining such a fuel.

Description

[Technical Field]

[0001] The field of this invention is the field of fuels for compression-ignition engines, particularly engines for maritime applications. The fuel of this invention is one of a new generation of fuels that have a reduced environmental impact, such as low-carbon electricity, low-carbon hydrogen, and / or what is commonly called "e-fuel" when produced from CO2. These fuels are considered a solution for decarbonizing heavy cargo transport and long-distance transport. Therefore, the fuel of this invention, which has ammonia as its main component, is one of the environmentally friendly alternative fuels to diesel and heavy oil. This invention relates to an ammonia-based fuel that can be used with a pilot fuel to improve the ignition of the fuel in the engine and burn faster and more powerfully. [Background technology]

[0002] Ammonia (NH3) is an environmentally friendly alternative and a promising renewable fuel that could replace diesel and heavy oil in maritime transport in the near future. The combustion of NH3 produces only water and nitrogen; it does not generate carbon molecules (CO2, CO) or soot. Because ammonia is a commonly produced and proven compound, its uses, transportation, and storage methods are well-known. It also maintains a reasonable price for use as a fuel. As a fuel, ammonia has a usable energy density in compression-ignition engines, but its cetane number is low. Ignition in diesel engines remains challenging, especially at low engine speeds. Ammonia is stored in tanks in liquid form under pressure (approximately 9 bar) and injected into engines in liquid or gaseous form.

[0003] Various methods have been described to improve the ignition of ammonia in compression ignition engines.

[0004] U.S. Patent Application Publication No. 2010 / 288211 describes a method for preparing a mixture of ammonia and heavy oil for use in an internal combustion engine, comprising: (i) supplying ammonia to a metering and mixing module; (ii) supplying heavy oil to a metering and mixing module; and (iii) mixing the supplied ammonia and heavy oil in a predetermined ratio in the metering and mixing module.

[0005] The scientific paper "Ignition and combustion study of premixed ammonia using GDI pilot injection in CI engine" (published in Fuel Volume 331, Part 1, 1 January 2023, 125768) states that n-heptane is useful as a pilot fuel containing ammonia. n-heptane was chosen because of its low viscosity (easy injection) and its cetane number (53-56) which is close to that of commercially available diesel fuel.

[0006] U.S. Patent Application Publication 2011 / 0259290 describes a combustion engine that operates on a mixture of ammonia and a highly flammable substance that burns more readily than ammonia. U.S. Patent Application Publication 2022 / 056856 describes a propulsion system comprising a piston engine capable of operating on ammonia (NH3) as the primary fuel, and a pilot ignition system configured to ignite a mixture of air and ammonia by igniting a pilot fuel containing hydrogen, biofuels, and / or fossil fuels.

[0007] The weight ratio of ammonia used to additional fuel is not specified in U.S. Patent Application Publication No. 2011 / 0259290 and U.S. Patent Application Publication No. 2022 / 056856.

[0008] On another note, in the field of hydrocarbon diesel or biodiesel fuels with low natural cetane numbers (generally close to 42-55), 2-ethylhexyl nitrate (EHN) has long been used as a cetane number improver in diesel fuels. A higher cetane number reduces fuel consumption, decreases particulate matter and NOx emissions, improves cold-start engine performance, reduces engine knocking and noise, and decreases engine wear. The reaction mechanism of EHN in the presence of hydrocarbon diesel fuels 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). This additive is industrially produced and widely used in commercially available diesel fuels. In Europe, more than 50,000 tons of EHN have been produced annually since the 1980s. Other alkyl nitrates can also be used as additives to improve the cetane number of diesel fuel. A cetane number improvement effect can be obtained when the concentration of alkyl nitrate added to diesel fuel is approximately 0.03% by weight.

[0009] When the fuel's inherent cetane number is already generally high, exceeding 60, fuel cetane number improvers such as alkyl nitrate are not useful. Furthermore, it should be noted that when the cetane number exceeds 67, measurement using standard methods compliant with EN 16715 remains difficult. [Overview of the project] [Means for solving the problem]

[0010] This disclosure provides a solution to improve the ignition and combustibility of ammonia in a standard engine by using a pilot fuel composed of highly reactive hydrocarbons (having a high cetane number greater than 60) containing alkyl nitrate additives such as 2-ethylhexyl nitrate (2-EHN or EHN) in a very small proportion of the fuel. The energy fraction (PEF) of the pilot fuel added to the ammonia main fuel is 0.1% to 5% of the total fuel (ammonia + pilot fuel).

[0011] The PEF of pilot fuel is calculated using the following formula.

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[0012] Since the lower heating value of pilot fuel depends on the properties of the pilot fuel, the weight ratio of pilot fuel to ammonia (equivalent to PEF) changes depending on the properties of the fuel, even with the same PEF. For example, when the PEF is 1% and the pilot fuel is HVO (an acronym for "Hydrotreated Vegetable Oil"), the weight ratio of HVO to ammonia is approximately 0.52%. It is unexpected that even with an extremely small energy proportion of the pilot fuel used, the ignition and combustibility of ammonia-based fuels can be improved, and even achieved, very effectively, both at warm and cold temperatures. A low addition rate of pilot fuel is advantageous in suppressing the emission of greenhouse gas combustion products. Therefore, the high-cetane hydrocarbons and alkyl nitrate additives that make up the pilot fuel work synergistically to improve the combustibility of ammonia. [Brief explanation of the drawing]

[0013] [Figure 1]This shows the pressure measured in the combustion chamber relative to the crankshaft angle (CA) when pilot fuel consisting of dodecane containing 0%, 1%, and 10% EHN by weight is injected at CA = -10°. [Figure 2] This shows the temperature measured in the combustion chamber relative to the crankshaft angle (CA) when pilot fuel consisting of dodecane containing 0%, 1%, and 10% EHN by weight is injected at CA = -10°. [Figure 3] This shows the amount of heat dissipated in the combustion chamber relative to the crankshaft angle (CA) when pilot fuel consisting of dodecane containing 0%, 1%, and 10% EHN by weight is injected at CA = -10°. [Figure 4] This shows the pressure measured in the combustion chamber relative to the crankshaft angle (CA) when pilot fuel consisting of dodecane containing 10% EHN is injected at CA = -10° during a cold start (30°C) and a hot start (80°C). [Figure 5] This shows the temperature measured in the combustion chamber relative to the crankshaft angle (CA) when pilot fuel consisting of dodecane containing 10% EHN is injected at CA = -10° and a cold start (30°C) and a hot start (80°C). [Figure 6] This shows the pressure measured in the combustion chamber relative to the crankshaft angle (CA) when pilot fuel composed of HVO containing 0%, 1%, and 10% EHN by weight is injected at CA = -10°. [Figure 7] This shows the temperature measured in the combustion chamber relative to the crankshaft angle (CA) when pilot fuel composed of HVO containing 0%, 1%, and 10% EHN by weight is injected at CA = -10°. [Figure 8] This shows the amount of heat dissipated in the combustion chamber relative to the crankshaft angle (CA) when pilot fuel composed of HVO containing 0%, 1%, and 10% EHN by weight is injected at CA = -10°. [Figure 9]Pilot fuel composed of HVO containing 30% by weight of EHN was injected at CA (crankshaft angle) = -10°, and the pressure measured in the combustion chamber with respect to CA during cold start (30°C) and hot start (80°C) is shown.

Mode for Carrying Out the Invention

[0014] The present disclosure relates to a fuel for a combustion engine that uses ammonia as the main fuel and adds pilot fuel, particularly for a compression ignition engine. The pilot fuel is composed of a hydrocarbon with a cetane number exceeding 60 and contains an additive composed of an alkyl nitrate or a mixture of alkyl nitrates in an amount of about 0.02 to about 30.0% by weight. In some embodiments, the hydrocarbon with a cetane number exceeding 60 contains an additive composed of an alkyl nitrate or a mixture of alkyl nitrates in an amount of about 0.5 to about 30.0% by weight, or about 1.0 to about 30.0% by weight, or about 1.0 to about 20.0% by weight, or about 1.0 to about 10.0% by weight.

[0015] As described above, the fuel for a combustion engine according to the present invention contains ammonia as the main fuel in an amount of about 95.00 to about 99.75% by weight.

[0016] As described below, the pilot fuel is injected into the combustion chamber of the ignition engine before or after the supply of ammonia. In some embodiments, the fuel (ammonia + pilot fuel) thus formed contains the pilot fuel in an amount of about 0.25 to about 5.0% by weight based on the total weight of the fuel (ammonia + pilot fuel). In some embodiments, when the total amount of ammonia and the amount of pilot fuel do not equal 100% by weight, the balance up to 100% is added to the fuel by one or more other additives such as additives having functions of lubricity, corrosion resistance, or detergency. Such additives may be included in the pilot fuel or separately introduced into the combustion chamber.

[0017] [[ID=十九]] The hydrocarbons in the pilot fuel are obtained by distilling petroleum or by synthesis from renewable raw materials. The pilot fuel may also be a pure substance belonging to the straight-chain or branched-chain alkanes.

[0018] In some embodiments, the hydrocarbon in the pilot fuel with a high cetane number (>60) is selected from at least one of the following compounds. • Diesel fuel with low aromatic content (<10%) and low polycyclic aromatic content (<10%); • Hydrogenated vegetable oil (HVO) (or renewable diesel from waste oil, or renewable diesel from animal fats), such as the HVO100 fuel type sold by TotalEnergies; • Isoalkanes, such as the Isane Biolife type isoparaffin mixture sold by TotalEnergies; n-alkanes such as dodecane; • Sustainable aviation fuel (SAF: Based on standards established by the "Carbon Offset and Reduction Scheme for International Aviation," which is part of ICAO / UN)

[0019] In some embodiments, hydrocarbons in the pilot fuel are selected from hydrogenated vegetable oils; n-alkanes, particularly dodecanes; and mixtures thereof.

[0020] Additives are present in pilot fuel at a concentration of approximately 0.02% to 30.0%, but they account for approximately 0.0002% to 1.5% of the total fuel weight.

[0021] The additive added to the pilot fuel is composed of an alkyl nitrate or a mixture of alkyl nitrates selected from linear, branched or cyclic alkyl nitrates. In some embodiments, the alkyl nitrate is a nitrate ester of a linear alkyl having 4 to 36 carbon atoms, preferably 4 to 24 carbon atoms, a nitrate ester of a branched alkyl having 4 to 36 carbon atoms, preferably 4 to 24 carbon atoms, a nitrate ester of a cyclic alkyl having 5 to 18 carbon atoms (or cycloalkyl nitrate), and mixtures thereof. In some embodiments, the alkyl nitrate is selected from 2-ethylhexyl nitrate, cyclohexyl nitrate, dodecyl nitrate, n-nonyl nitrate, 2-tetradecyl-1-octadecyl nitrate, isononyl nitrate, hexyl nitrate, 2-octyl nitrate, isononyl nitrate, 2-propylheptyl nitrate, C9-C 13 a mixture of nitrate esters of branched alkyls, and mixtures thereof. In some embodiments, the alkyl nitrate is 2-ethylhexyl nitrate alone or a mixture with one or more other alkyl nitrates as defined above. Advantageously, the alkyl nitrate is 2-ethylhexyl nitrate alone.

[0022] C9-C 13 The mixture of nitrate esters of branched alkyls is obtained from the corresponding C9-C 13 mixture of branched alcohols, for example alcohols available under the trade name Exxal from Exxon TM After preparing a mixture of at least two branched alcohols selected from C9 branched alcohol, C 10 branched alcohol, C 11 branched alcohol, C 12 branched alcohol, and C 13 branched alcohol, the corresponding mixture of alkyl nitrates can be synthesized.

[0023] A fuel comprising ammonia, pilot fuel, and optional additives can be used in engines (particularly combustion engines) such as those in vehicles (automobiles, trucks, tractors, etc.) or ships (oil tankers, container ships, etc.). Accordingly, one aspect of this disclosure relates to a vehicle or ship engine containing the fuel as defined herein. This disclosure also relates to a vehicle or ship having the engine as defined herein.

[0024] Ammonia is typically injected into the engine's combustion chamber in liquid form, or drawn into the combustion chamber in gaseous form, and may or may not be premixed with air upon intake.

[0025] The pilot fuel and ammonia (in liquefied state) are stored separately. The pilot fuel is injected into the combustion chamber before or after the addition of ammonia (more specifically, after the addition of gaseous ammonia premixed with air, as in Examples 1 and 2 below), or it is injected simultaneously with the ammonia by a dual injection system.

[0026] Therefore, according to one embodiment, the present disclosure relates to a method for obtaining a fuel as defined herein, comprising the step of mixing a pilot fuel (as defined herein) with gaseous ammonia or liquefied ammonia, premixed or unmixed with intake air, in the combustion chamber of an engine. In some embodiments, the pilot fuel is injected into the combustion chamber simultaneously with ammonia by a dual injection system. In some embodiments, the pilot fuel is injected into the combustion chamber before the intake or injection of ammonia. In some embodiments, the pilot fuel is injected into the combustion chamber after the intake or injection of ammonia.

[0027] Injecting pilot fuel makes it easier to ignite ammonia, but this can be difficult, or even impossible, under certain engine conditions (fuel mixture richness, temperature, load, etc.).

[0028] Using the pilot fuel as defined herein improves ignition and combustion performance and facilitates cold ignition compared to ammonia alone.

[0029] Accordingly, in one embodiment, the Disclosure relates to the use of a pilot fuel as defined herein, which is contained in an amount of 0.25 to 5.0% by weight in a fuel containing about 95.00 to about 99.75% by weight of ammonia, as an ignition accelerator for the said fuel. The Disclosure also relates to a method for improving the ignitionability of a fuel containing about 95.00 to about 99.75% by weight of ammonia in an engine, the method comprising the step of adding a pilot fuel as defined herein to ammonia in an amount of 0.25 to 5.0% by weight.

[0030] In one aspect, the present disclosure relates to a vehicle or marine engine that includes (operates with) the fuel defined above.

[0031] In one aspect, the disclosure relates to a vehicle or vessel having the engine defined above.

[0032] The following examples illustrate this disclosure, but they are for illustrative purposes only. [Examples]

[0033] Fuel performance tests relating to this disclosure were conducted using a reactivity-controlled compression ignition engine ("RCCI" type). When the engine is operated in RCCI mode, a low-reactivity fuel is ignited by the energy input of a high-reactivity fuel. The low-reactivity fuel, which is indirectly injected or inhaled, enters the engine during the intake phase. The high-reactivity fuel, which is present in small amounts in the entire mixture, is injected directly into the cylinder in one or more injections, similar to a compression ignition engine. In this case, the low-reactivity fuel and air are premixed during intake (the mixing ratio is determined at this time), and a small amount of high-reactivity fuel is directly injected one or more times to ignite it. As a result, each point such as the premixing ratio, the state of the low-reactivity mixture (temperature, pressure), and the injection of the high-reactivity fuel (pressure, time, timing) can be controlled individually.

[0034] Research using RCCI engines has shown that the responsiveness of fuel charging can be appropriately defined, and optimal operation can be achieved by adjusting its relative amount, synchronization timing, and combustion.

[0035] The experimental data collected for various examples are shown against the applied indicated mean pressure (IMP) and mixing ratio (air / NH3).

[0036] The illustrated mean pressure (IMP) corresponds to the average pressure inside the cylinder due to fuel combustion, and it represents the amount of work produced by this combustion relative to the engine's displacement.

[0037] The following are experimental data measured inside the combustion chamber. • P: Pressure in the combustion chamber • T: Temperature inside the combustion chamber Q: Heat generated by combustion

[0038] These experimental data are reported as a function of the crankshaft angle (CA = 0° ⇔ Top Dead Center (TDC)).

[0039] In Examples 1 and 2, after introducing inhaled gaseous ammonia premixed with air, pilot fuel is injected at a CA angle of -10°.

[0040] Example 1 In this embodiment, the pilot fuel is dodecane (cetane number > 67) alone (2-ethylhexyl nitrate: 0%), or the above dodecane containing 1% or 10% by mass of 2-ethylhexyl nitrate (EHN). The main fuel is ammonia.

[0041] The amount of pilot fuel injected corresponds to a PEF (Pilot Fuel) that is 2% of the total energy input of the ammonia + pilot fuel mixture. The weight percentage of pilot fuel in the total fuel (ammonia + pilot fuel) is 0.9%.

[0042] Steady-state operation at warm temperature (intake temperature = 80°C) The engine's operating point corresponds to an IMP of 3 bar at an intake temperature of 80°C and an air / NH3 mixture ratio Φ=0.7. Experimental data will be collected after at least 200 engine operating cycles.

[0043] Under these operating conditions, ignition will not occur if ammonia is used alone without pilot fuel.

[0044] The following can be seen from Figures 1, 2, and 3. When the pilot fuel consists of dodecane without EHN, the first pressure peak occurs approximately at TDC (compression of the mixture, CA=0°), the second pressure peak occurs around 20°, and the heat dissipation peak and temperature rise peak due to the start of combustion are observed at an angle of approximately CA=5°. If the pilot fuel consists of dodecane containing 1% by weight of EHN, the pressure increases continuously from an angle of approximately CA=0°. A very slight flat is observed, but the pressure continues to rise up to an angle of approximately CA=20°. Combustion begins at an angle of approximately CA=0°, generating heat and raising the temperature. When the pilot fuel consists of dodecane containing 10% by weight of EHN, a continuous pressure increase is observed following the pressure peak due to compression, reaching a higher level than that seen with pilot fuel containing 1% EHN. Therefore, combustion begins at an angle of approximately CA = -2.5°. This earlier onset of heat generation can also be seen in the temperature and heat dissipation curves as a function of CA.

[0045] In conclusion, injecting a highly reactive alkane pilot fuel (high cetane number) containing up to 10% by weight of EHN (dodecane in this embodiment) significantly improves ignition delay, flammability, and heat generation.

[0046] Similar tests conducted using dodecane pilot fuel containing 30% EHN by weight showed no significant difference compared to tests conducted using dodecane pilot fuel containing 10% EHN by weight.

[0047] Cold start and hot start The advantages of the pilot fuel described herein are also demonstrated with respect to the fuel's cold ignition properties. The engine operating point corresponds to an IMP of approximately 8.5 bar at an intake temperature of 80°C when warm and 30°C when cold, with an air / NH3 mixture ratio of Φ=1. The fuel's PEF is 2%.

[0048] Under these operating conditions, when cold and with an intake temperature of 30°C, combustion will not start under the following circumstances. • Ammonia alone • Pilot fuel consisting solely of ammonia and dodecane (EHN not included) • Pilot fuel consisting of dodecane containing 1% by weight of ammonia + EHN

[0049] Referring to Figures 4 and 5, combustion initiation is observed under both warm and cold conditions when using dodecane pilot fuel containing 10% by weight of EHN.

[0050] Comparing cold operation and warm operation in Figures 4 and 5, the following can be observed. Even with a low peak heat output, combustion lasts longer and more energy is recovered during the cycle, resulting in a higher IMP (Intensity Potential) when the engine is cold. • Combustion starts later when the engine is cold (CA = 5° angle) than when it is warm (CA = 0° angle). • Combustion pressure and temperature are lower when the engine is cold.

[0051] Similar tests conducted using dodecane pilot fuel containing 30% EHN by weight showed no significant difference compared to tests conducted using dodecane pilot fuel containing 10% EHN by weight.

[0052] In conclusion from Example 1, when a pilot fuel composed of dodecane contains 10% by weight of EHN and is present in the total fuel at a low PEF of 2% (corresponding to a weight ratio of 110-1200 ppm relative to the total fuel (ammonia + pilot fuel)), engine performance can be significantly improved in both hot and cold conditions.

[0053] In this embodiment, increasing the EHN content to more than 10% by weight does not lead to a significant improvement in performance.

[0054] Tables 1 and 2 summarize the results of Example 1.

[0055] [Table 1]

[0056] [Table 2]

[0057] Example 2 In this embodiment, the pilot fuel is hydrogenated vegetable oil HVO100 (cetane number > 60) sold by TotalEnergies, alone (2-ethylhexyl nitrate: 0%), or the above HVO100 containing 1% or 10% by weight of 2-ethylhexyl nitrate (EHN). The main fuel is ammonia.

[0058] The amount of pilot fuel injected corresponds to a PEF (Pilot Fuel Output) that is 2% of the total energy input of the mixture (ammonia + pilot fuel). The weight percentage of pilot fuel in the total fuel (ammonia + pilot fuel) is 1.1%.

[0059] Steady-state operation (intake temperature = 80°C) The engine's operating point corresponds to an IMP of 3 bar at an intake temperature of 80°C and an air / NH3 mixture ratio Φ=0.7. Experimental data will be collected after at least 200 engine operating cycles.

[0060] From Figures 6, 7, and 8, the following can be seen. When the pilot fuel consists of HVO without EHN, the first pressure peak occurs at approximately CA=0° near TDC, the second pressure peak occurs at approximately CA=20°, and the heat dissipation peak and temperature rise peak due to the start of combustion are observed at approximately CA=5°. When the pilot fuel consists of HVO containing 1% by weight of EHN, the combustion curve of the fuel is similar to that obtained with pilot fuel consisting of HVO without EHN. With pilot fuel consisting of HVO containing 1% by weight of EHN, slight but significant improvements are obtained in terms of ignition advance, thermal pressure peak, and temperature peak. When the pilot fuel consists of HVO containing 10 wt% EHN, a continuous pressure increase is observed following the pressure peak due to compression, reaching a higher level than that seen with the pilot fuel containing 1% EHN. Combustion begins at an angle of approximately CA = -2.5°. This earlier onset of heat generation can also be seen in the temperature and heat dissipation curves as a function of CA.

[0061] In conclusion, injecting a pilot fuel composed of HVO (cetane number > 67) containing 10% EHN significantly improves combustion initiation delay and heat generation.

[0062] Similar tests conducted using a pilot fuel composed of HVO containing 30% EHN by weight showed no significant difference compared to tests conducted using a pilot fuel composed of HVO containing 10% EHN by weight.

[0063] Cold start and hot start The advantages of the pilot fuel described herein are also demonstrated with respect to the fuel's cold ignition properties. The engine operating point corresponds to an IMP of approximately 8.5 bar at an intake temperature of 80°C when warm and 30°C when cold, with an air / NH3 mixture ratio of Φ=1. The fuel's PEF is 2%.

[0064] Under these operating conditions, when cold and with an intake temperature of 30°C, combustion will not start under the following circumstances. • Ammonia alone • Pilot fuel consisting solely of ammonia + HVO (EHN not included) • Pilot fuel consisting of HVO containing 1% by weight of ammonia + EHN • Pilot fuel consisting of HVO containing 10% by weight of ammonia + EHN

[0065] Referring to Figures 9 and 10, combustion initiation is observed under both warm and cold conditions when using a pilot fuel composed of HVO containing 30% by weight of EHN.

[0066] Comparing cold operation and warm operation in Figures 9 and 10, the following can be observed. • Combustion starts later when the CA angle is approximately 5° compared to when the CA angle is approximately 0° (warm), and if injection is simultaneous, the combustion time is shorter.

[0067] In conclusion from Example 2, engine performance is significantly improved when the pilot fuel consists of HVO containing at least 10% EHN and is present in the total fuel at a low PEF of 2% (corresponding to a weight ratio of 0.14% of the total fuel (ammonia + pilot fuel)).

[0068] Tables 3 and 4 summarize the results of Example 2.

[0069] [Table 3]

[0070] [Table 4]

Claims

1. A fuel for a combustion engine, comprising 95.00 to 99.75% by weight of ammonia and 0.25 to 5.00% by weight of pilot fuel, wherein the pilot fuel is composed of hydrocarbons with a cetane number exceeding 60 and contains 0.02 to 30.0% by weight of alkyl nitrate or a mixture of alkyl nitrates.

2. The fuel according to claim 1, wherein the pilot fuel contains 0.5 to 30.0% by weight, preferably 1.0 to 30.0% by weight, more preferably 1.0 to 20.0% by weight, and most preferably 1.0 to 10.0% by weight, of alkyl nitrate or a mixture of alkyl nitrate.

3. The fuel according to claim 1 or 2, wherein the hydrocarbon is selected from at least one compound among diesel fuel with a low aromatic content (<10%) and a low polycyclic aromatic content (<10%), hydrogenated vegetable oil (HVO), isoalkane, n-alkane, and sustainable aviation fuel.

4. The fuel according to any one of claims 1 to 3, wherein the alkyl nitrate is a nitrate ester of a linear alkyl having 4 to 36 carbon atoms, a nitrate ester of a branched alkyl having 4 to 36 carbon atoms, a nitrate ester of a cyclic alkyl having 5 to 18 carbon atoms, or a mixture thereof.

5. The alkyl nitrates listed above are 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, and C ９ ~C １３ A mixture of branched-chain alkyl nitrate esters, and a fuel according to any one of claims 1 to 4, selected from these mixtures.

6. The fuel according to any one of claims 1 to 5, wherein the alkyl nitrate is 2-ethylhexyl nitrate.

7. A method for obtaining the fuel described in any one of claims 1 to 6, comprising the step of mixing the pilot fuel with gaseous ammonia or liquefied ammonia that is premixed or not premixed with intake air in the combustion chamber of the engine.

8. The method according to claim 7, wherein the pilot fuel is injected into the combustion chamber simultaneously with the ammonia by a dual injection system.

9. The method according to claim 7, wherein the pilot fuel is injected into the combustion chamber before ammonia is suctioned or injected.

10. The method according to claim 7, wherein the pilot fuel is injected into the combustion chamber after the intake or injection of ammonia.

11. A vehicle or marine engine comprising a fuel as defined in any one of claims 1 to 6.

12. A vehicle or vessel having the engine defined in claim 11.

13. A pilot fuel containing 0.25 to 5.0% by weight of ammonia in a fuel containing 95 to 99.75% by weight of ammonia, comprising a hydrocarbon with a cetane number exceeding 60, and containing 0.02 to 30.0% by weight of alkyl nitrate or a mixture of alkyl nitrate, for use as an ignition accelerator for the above ammonia-based fuel.

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