A Review of Current Advances in Ammonia Combustion from the Fundamentals to Applications in Internal Combustion Engines

: The energy transition from hydrocarbon-based energy sources to renewable and carbon-free energy sources such as wind, solar and hydrogen is facing increasing demands. The decarbonization of global transportation could come true via applying carbon-free fuel such as ammonia, especially for internal combustion engines (ICEs). Although ammonia has advantages of high hydrogen content, high octane number and safety in storage, it is uninﬂammable with low laminar burning velocity, thus limiting its direct usage in ICEs. The purpose of this review paper is to provide previous studies and current research on the current technical advances emerging in assisted combustion of ammonia. The limitation of ammonia utilization in ICEs, such as large minimum ignition energy, lower ﬂame speed and more NO x emission with unburned NH 3 , could be solved by oxygen-enriched combustion, ammonia–hydrogen mixed combustion and plasma-assisted combustion (PAC). In dual-fuel or oxygen-enriched NH 3 combustion, accelerated ﬂame propagation speeds are driven by abundant radicals such as H and OH; however, NOx emission should be paid special attention. Furthermore, dissociating NH 3 in situ hydrogen by non-noble metal catalysts or plasma has the potential to replace dual-fuel systems. PAC is able to change classical ignition and extinction S-curves to monotonic stretching, which makes low-temperature ignition possible while leading moderate NO x emissions. In this review, the underlying fundamental mechanism under these technologies are introduced in detail, providing new insight into overcoming the bottleneck of applying ammonia in ICEs. Finally, the feasibility of ammonia processing as an ICE power source for transport and usage highlights it as an appealing choice for the link between carbon-free energy and power demand.


Introduction
Global transition from traditional fossil to renewable resources has been a concern for years to mitigate greenhouse gases.To a large extent, renewable fuels are regarded as promising energy carries particularly adapted for long-distance and high-powered mobility or long-term energy storage.Hydrogen, a carbonless fuel, has been recognized as the most promising fuel and clean energy carrier for automotive, marine and power generation [1].A global hydrogen-based economy will be a sunrise for the energy issue after solving the bottleneck regarding the transportation and storage of hydrogen with reliable safety and reasonable cost.Therefore, closer attention has been paid to studying H 2 -carrier fuel in the form of different chemical substances.
Ammonia (NH 3 ) is presently receiving a surge of attention as an energy carrier of high gravimetric hydrogen density (17.8% hydrogen content by mass), and is considered a carbon-free fuel that can be directly used in both combustion and fuel cell systems.Ammonia is a colorless gas with a very pungent odor at room temperature and can be dissolved in water.Ammonia has trigonal pyramidal molecule geometry with three hydrogen atoms, as well as an unshared pair of electrons attached to a nitrogen atom, and Energies 2023, 16, 6304 2 of 20 molecule polar covalent bond formation takes place between nitrogen and hydrogen [2].Due to the strong hydrogen bonds between ammonia molecules, it is easily liquefied.Liquefaction of ammonia is able to happen at 10 bar at room temperature or −33.4 • C at atmospheric pressure, while hydrogen can only be liquefied under −253 • C. Ammonia is able to be stored in liquid form under suitable pressure conditions, ensuring a comparable energy density with other fuels, and it has competitive lower heating value (LHV) [3].In terms of energy density in storage conditions, liquid ammonia is more than 40% denser than liquid hydrogen or more than twice that of compressed gas hydrogen.
Ammonia is the second-largest chemical products (after sulfuric acid) in the world (over 200 million tons per annum globally with more than USD $60 billion market value) [4,5].Approximately 80% of global ammonia is utilized in agriculture as fertilizer, with around 5% for explosives, and the balance for industrial cooler refrigerant and chemical commodities.Currently, little ammonia is utilized for energy carriage, but there is definitely great potential for ammonia to be consumed as a renewable fuel in gas turbines, internal combustion engines or fuel cells without a carbon footprint in the years to come.The industrial production of ammonia (NH 3 ) from N 2 and H 2 is mainly dominated by the Haber-Bosch (H-B) process in the presence of metal catalyst at high temperature (~700 K) and 10-25 MPa, responsible for 1.2% of the global anthropogenic CO 2 emissions [6].The energy consumption of green H-B is within the range of 27.4-31.8GJ t NH3 −1 , and improvement in overall energy efficiency up to 65% [7].Furthermore, ammonia could be manufactured from renewable energy sources such as biomass, wind or solar.Achieving a CO 2 -free, energy-efficient, low-capital Haber-Bosch synthesis loop is under investigation by electrically driven [8] or electrochemical [9] power sources.In addition, because the absorption and desorption reactions of ammonia is fully reversible, ammonia is able to be released from a solid complex such as Mg(NH 3 ) 6 Cl 2 upon heating and compacted into a dense shape, which makes storage simple and safe [10].Some of the metal ammine complexes show promising results for storing hydrogen, for example, Mg(NH 3 ) 6 Cl 2 can store over 9% hydrogen by weight in its solid form.Therefore, ammonia is considered a superb fuel due to its carbon-free structure, safe storage and transportation and low production cost, but with high hydrogen gravimetric density.
Ammonia has regained a great deal of interest from governments and institutes, since it not only enables the vital delivery of nitrogen needed for crop growth but also serves as a chemical that is capable of producing cooling, heating, power, and propulsion with minimum storage cost [11].When ammonia is labeled as one of the carbon-free alternative fuels, the interest in deploying ammonia is in fast growth.Herbinet et al. [12] summarized an interesting comparison between the advantages and disadvantages of ammonia when used as power solutions.Gas turbines are the system of choice for large-scale ammonia utilization, while solid oxide fuel cells perform better at small scales (below 10 MW).Ammonia can also be directly used in both spark ignition engines or compression ignition engines [12,13].Predicted by life cycle analysis, it will reduce greenhouse gas emissions by three times through ammonia-fueled vehicles as an alternative to conventional gasoline [14].The UK Department for Transport proposed taking action on clean maritime growth by placing "a group of hydrogen or ammonia powered domestic vessels in operation" [15].Indeed, ammonia has a higher octane number than gasoline and natural gas (Table 1), thus allowing a higher compression ratio applied in engine operation.Even though the energy density of compressed ammonia is comparably less than that of gasoline and diesel, it is still much higher than that of compressed natural gas or liquid hydrogen.Research on ammonia for internal combustion engines (ICEs) is in its early stages.Ammonia has extreme low combustion intensity with difficult flammability, reflected by its narrow flammability limits and low laminar burning velocity (Table 1).

Method of Ammonia Production
Ammonia production can be classified as brown (or gray) ammonia, blue ammonia and green ammonia based on the carbon emissions from the manufacturing process [18].Current brown ammonia is massively produced by reforming hydrocarbons such as methane using proven Haber-Bosch industrial technology [19], which requires a high-temperature, high-pressure environment as well as addition of N 2 , as shown in Figure 1.Blue ammonia is able to be generated using hydrogen from natural gas in conjunction with CCUS (carbon capture, utilization, and storage), while the most desirable green ammonia can be made using hydrogen from water electrolysis without CO 2 emissions.Overall, the majority of ammonia (around 86% to 96%) is manufactured through the traditional Haber-Bosch process, although it is energy-intensive with low conversion efficiency (25% at 30 MPa) and high carbon emissions [20].In addition, the remaining commercial ammonia comes from alternative technologies such as electrochemical or thermochemical processes etc. [19,20] powered by low pressure and no catalysts with carbon-free emissions [21].Moreover, the electrochemical process can be performed over a wide range of temperatures, depending on the electrolyte applied during the process [19].
underlying kinetic processes of plasma-assisted ammonia synthesis.They both found surface interactions such as Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reactions contributed to the production of ammonia significantly.The dominant reaction mechanisms of plasma-catalyzed ammonia synthesis are illustrated in Figure 2 and were summarized by Engelmann et al. [25].Due to electron collisions, a large number of N2 and H2 molecules were ionized, excited, adsorbed and dissociated, followed by the adsorption of free radicals (N, H, NH and NH2) on the surface of the catalyst.After that, these particles in the adsorbed state either underwent E-R reactions with groups in the gas phase or LH reactions on the catalyst surface due to atomic diffusion.Eventually, the resulting NH3 was released from the catalyst surface.Unlike the traditional H-B process, breakage of the NN triple bond can happen at room temperature and pressure via plasma-assisted ammonia synthesis.However, the efficiency of plasma-assisted ammonia synthesis alone is not high because the strength of the NN bond (NN bond = 946 kJ/mol) is much greater than that of the NH bond (NH bond = 389 kJ/mol) at room temperature.Therefore, the choice of proper catalysts to improve the selectivity of the reaction products is also a hot topic of research.
An efficient catalyst should exhibit not only strong adsorption of reactants but also low dissociation energy of products, which is contradictory to the catalyst itself.However, it is possible to modulate the bonding strength of N2 and catalyst by elaborate design of alloy catalysts based on the surface bonding energy between metal atoms and N atoms.Xie et al. [26] prepared a high-entropy alloy (HEA) containing five common metallic elements that became saturated at 600 °C (with 100% conversion), and it was much more effective in catalysis than the noble metal Ru (with 73% conversion).Plasma-assisted ammonia synthesis is alternative option to produce NH 3 from H 2 and N 2 via the formation of NH radicals.Many studies have concentrated on the application of plasma catalysts at ambient temperature.Li et al. [22] found that the reaction rate of ammonia synthesis was facilitated by the interaction between catalyst and plasma, which was double the pure catalyst load and 30 times that of plasma only.Andersen et al. [23] and Sun et al. [24] used the open-source code ZDPlasKin to numerically figure out the underlying kinetic processes of plasma-assisted ammonia synthesis.They both found surface interactions such as Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reactions contributed to the production of ammonia significantly.The dominant reaction mechanisms of plasma-catalyzed ammonia synthesis are illustrated in Figure 2 and were summarized by Engelmann et al. [25].Due to electron collisions, a large number of N 2 and H 2 molecules were ionized, excited, adsorbed and dissociated, followed by the adsorption of free radicals (N, H, NH and NH 2 ) on the surface of the catalyst.After that, these particles in the adsorbed state either underwent E-R reactions with groups in the gas phase or LH reactions on the catalyst surface due to atomic diffusion.Eventually, the resulting NH 3 was released from the catalyst surface.Unlike the traditional H-B process, breakage of the NN triple bond can happen at room temperature and pressure via plasma-assisted ammonia synthesis.However, the efficiency of plasma-assisted ammonia synthesis alone is not high because the strength of the NN bond (NN bond = 946 kJ/mol) is much greater than that of the NH bond (NH bond = 389 kJ/mol) at room temperature.Therefore, the choice of proper catalysts to improve the selectivity of the reaction products is also a hot topic of research.

Ignition Energy
The minimum ignition energy (MIE) is typically defined as the minimum energy required to ignite the combustible gas [27], and a lower MIE indicates that a stable fire nucleus can be formed more easily in the early stages of combustion.Although ammonia has a high hydrogen content, pure ammonia combustion is inherently difficult to ignite.An ignition energy of 2.8 J is still required in spite of mixing with hydrogen fuel under a modified ignition system, which is two orders of magnitude higher than hydrocarbon fuel and four orders of magnitude higher than hydrogen [28].Xin et al. [29] performed experimental studies on combustion and emission properties of a hydrogen/ammonia-fueled engine at part-load operating conditions.As illustrated in Figure 3a,b, the addition of ammonia changed the combustion characteristics by prolonging ignition delay times (duration of CA0-10) and flame development periods (duration of CA10-90) due to its higher ignition energy.Meanwhile, the combustion phase was still controllable by modulation of ignition timing for improved indicated thermal efficiency (ITE) and acceptable NOx emission.An efficient catalyst should exhibit not only strong adsorption of reactants but also low dissociation energy of products, which is contradictory to the catalyst itself.However, it is possible to modulate the bonding strength of N 2 and catalyst by elaborate design of alloy catalysts based on the surface bonding energy between metal atoms and N atoms.Xie et al. [26] prepared a high-entropy alloy (HEA) containing five common metallic elements that became saturated at 600 • C (with 100% conversion), and it was much more effective in catalysis than the noble metal Ru (with 73% conversion).

Ignition Energy
The minimum ignition energy (MIE) is typically defined as the minimum energy required to ignite the combustible gas [27], and a lower MIE indicates that a stable fire nucleus can be formed more easily in the early stages of combustion.Although ammonia has a high hydrogen content, pure ammonia combustion is inherently difficult to ignite.An ignition energy of 2.8 J is still required in spite of mixing with hydrogen fuel under a modified ignition system, which is two orders of magnitude higher than hydrocarbon fuel and four orders of magnitude higher than hydrogen [28].Xin et al. [29] performed experimental studies on combustion and emission properties of a hydrogen/ammonia-fueled engine at part-load operating conditions.As illustrated in Figure 3a,b, the addition of ammonia changed the combustion characteristics by prolonging ignition delay times (duration of CA0-10) and flame development periods (duration of CA10-90) due to its higher ignition energy.Meanwhile, the combustion phase was still controllable by modulation of ignition timing for improved indicated thermal efficiency (ITE) and acceptable NOx emission.

Ignition Energy
The minimum ignition energy (MIE) is typically defined as the minimum energy required to ignite the combustible gas [27], and a lower MIE indicates that a stable fire nucleus can be formed more easily in the early stages of combustion.Although ammonia has a high hydrogen content, pure ammonia combustion is inherently difficult to ignite.An ignition energy of 2.8 J is still required in spite of mixing with hydrogen fuel under a modified ignition system, which is two orders of magnitude higher than hydrocarbon fuel and four orders of magnitude higher than hydrogen [28].Xin et al. [29] performed experimental studies on combustion and emission properties of a hydrogen/ammonia-fueled engine at part-load operating conditions.As illustrated in Figure 3a,b, the addition of ammonia changed the combustion characteristics by prolonging ignition delay times (duration of CA0-10) and flame development periods (duration of CA10-90) due to its higher ignition energy.Meanwhile, the combustion phase was still controllable by modulation of ignition timing for improved indicated thermal efficiency (ITE) and acceptable NOx emission.

Flame Speed
The primary limitation on the practical usage of ammonia as an engine fuel is the relatively slow flame speeds.The laminar burning velocity (LBV or Su) is an important parameter to characterize the premixed combustion process.Investigations on accelerating NH 3 efficient combustion with fuel additives have attracted the interest of many researchers.Numerical studies on the performance of premixed combustion of NH 3 /H 2 /air mixtures was conducted by Wang et al. [28] using Cantera open-source code.It was shown that the properties of NH 3 /H 2 /air mixture combustion could be comparable to that of hydrocarbon fuels under engine-relevant conditions; therefore, a high compression ratio is tolerable because of the excellent knock-resistance ability of the NH 3 /H 2 mixture.In addition, the increase in Su with H 2 addition contributes to facilitated diffusion, intensified reactivity, and increased flame temperature.At a compression ratio of 10, Su is observably improved with more H 2 added.However, for the stoichiometric combustion of NH 3 /H 2 , there still needs to be a 39% hydrogen doping ratio to reach a comparable Su level as CH 4 combustion.
Lhuillier et al. [30] found the same phenomenon when studying ammonia blends under engine-relevant turbulent conditions: enrichment of hydrogen in ammonia leads to an earlier, more intense heat release.To figure out the effects of additives such as H 2 , CO and CH 4 on the Su of ammonia blend combustion, Han et al. [31] proposed normalized enhancement parameters for quantitative analysis of the enhancement level.It was concluded that the effects of H 2 enhancement were exponential, while non-monotonic with CO and almost linear with CH 4 at mixing ratios greater than 0.2.Very low Su is constrained with pure CO/air combustion since almost no H or OH radical is accumulated during combustion, as shown in Figure 4, but when blended with moderate CO in NH 3 , a rapid increase in Su is achieved by decomposed H and OH radicals.The trends of Su almost coincided with the curves of the maximum H molar fraction, which implied the effect of H radicals on Su was dominant.Similar findings with concentration of H and OH radicals were also reported in the flame combustion of ammonia/methane/air [32].

Flame Speed
The primary limitation on the practical usage of ammonia as an engine fuel is the relatively slow flame speeds.The laminar burning velocity (LBV or Su) is an important parameter to characterize the premixed combustion process.Investigations on accelerating NH3 efficient combustion with fuel additives have attracted the interest of many researchers.Numerical studies on the performance of premixed combustion of NH3/H2/air mixtures was conducted by Wang et al. [28] using Cantera open-source code.It was shown that the properties of NH3/H2/air mixture combustion could be comparable to that of hydrocarbon fuels under engine-relevant conditions; therefore, a high compression ratio is tolerable because of the excellent knock-resistance ability of the NH3/H2 mixture.In addition, the increase in Su with H2 addition contributes to facilitated diffusion, intensified reactivity, and increased flame temperature.At a compression ratio of 10, Su is observably improved with more H2 added.However, for the stoichiometric combustion of NH3/H2, there still needs to be a 39% hydrogen doping ratio to reach a comparable Su level as CH4 combustion.
Lhuillier et al. [30] found the same phenomenon when studying ammonia blends under engine-relevant turbulent conditions: enrichment of hydrogen in ammonia leads to an earlier, more intense heat release.To figure out the effects of additives such as H2, CO and CH4 on the Su of ammonia blend combustion, Han et al. [31] proposed normalized enhancement parameters for quantitative analysis of the enhancement level.It was concluded that the effects of H2 enhancement were exponential, while non-monotonic with CO and almost linear with CH4 at mixing ratios greater than 0.2.Very low Su is constrained with pure CO/air combustion since almost no H or OH radical is accumulated during combustion, as shown in Figure 4, but when blended with moderate CO in NH3, a rapid increase in Su is achieved by decomposed H and OH radicals.The trends of Su almost coincided with the curves of the maximum H molar fraction, which implied the effect of H radicals on Su was dominant.Similar findings with concentration of H and OH radicals were also reported in the flame combustion of ammonia/methane/air [32].

NOx and Unburned NH3 Emissions
The use of ammonia fuels is effective in reducing hydrocarbon emissions, but relatively it also increases the content of NOx and unburned NH3 in exhaust.Representing significant progress in microscopic combustion kinetic reactions, chemical kinetic mechanisms are widely used to understand how quickly or slowly chemical reactions occur in nature.Numerous robust and concise chemical kinetic mechanisms have been proposed

NOx and Unburned NH 3 Emissions
The use of ammonia fuels is effective in reducing hydrocarbon emissions, but relatively it also increases the content of NOx and unburned NH 3 in exhaust.Representing significant progress in microscopic combustion kinetic reactions, chemical kinetic mechanisms are widely used to understand how quickly or slowly chemical reactions occur in nature.Numerous robust and concise chemical kinetic mechanisms have been proposed for ammonia oxidation under a wide range of conditions after verification of combustion characteristics and NOx emission.Since ammonia chemistry is less complex than that of hydrocarbons, the most recent ammonia oxidation mechanisms in different studies are generally fairly compatible.However, the differences are largely limited to the choice of rate constants or branching ratios for specific elementary reaction groups in varied application circumstances [33].It is concluded that NO generation in burning lean ammonia depends heavily on OH radicals and HNO relative reactions, but under rich-ammonia combustion, rich H radicals promote formation of NH x radicals [34].However, as concluded by Li et al. [35], development of an accurate mechanism to model ammonia-based flame is urgently necessary.Apart from NOx emission, the carbon capture from low-carbon fuel-assisted ammonia flame is still worth studying.
Wang et al. [28] studied the performance of premixed hydrogen-ammonia combustion by simulations.The integrated rate of production (ROP) of NO and the species molar fraction with varied hydrogen molar fraction α are displayed in Figure 5. NO was considered the main source of NOx and two competitive mechanisms of NO production were analyzed: on the one hand, as more H content involved, the flame temperature and concentrations of reactive O/H radical were both increased, leading to a growth in NO generation as well as NH 3 conversion; on the other hand, the decrease in the reactant NH 3 suppressed the NO-related reaction rates in turn.Unlike fuel NO, which was formed with NH 3 , the hot NO was more likely to be formed with the increasing of flame temperature caused by O/H radicals.Moreover, NO 2 was rapidly generated by conversion from NO in the flame region, then reconverted back to NO in the post-flame region via NO 2 + O→NO + O 2 , which finally reduced the total amount of NO 2 [36,37].Jin et al. [38] investigated the effect of different ammonia-to-energy ratios (AER) on the combustion and emission characteristic of an ammonia-diesel dual-fuel engine.Compared to pure diesel engines, the NO emission was decreased instead with increasing AER because of the denitrification of amine, forming more stable N 2 via reduction reaction.However, the incomplete combustion of NH 3 was increased significantly due to the low combustion temperature.As mentioned above, obstacles to the further development of ammonia-fueled engines are large minimum ignition energy, lower flame speed, and more NOx emission with unburned NH 3 .However, some new technologies expanded on the next chapter are expected to overcome the difficulties.

Oxygen-Enriched Combustion
It has been confirmed that oxygen-enriched combustion has the ability to lower fuel ignition points, speed up reactions, widen combustion limits and raise flame temperature.Therefore, some scholars studied the combustion characteristics of ammonia under oxygenenriched conditions.For internal combustion engines, the advantages of oxygen-enriched technology solve the problems of further oxidation of CO and unburned hydrocarbons, but bring higher NOx emissions [39][40][41][42][43][44].In order to overcome the shortcomings caused by oxygen enrichment, Liang et al. [45] applied an emulsification technique to diesel, which made the fuel unstable and then disperse throughout the combustion chamber after microexplosion during the compression stroke in the engine.As a result, the combination of water diesel emulsion and oxygen-enriched combustion reduced the combustion temperature in the cylinder, thus leading to less NOx emission, while the output power of the engine was lower than the normal level.Karimi et al. [46] investigated the effects of oxygen enrichment on the combustion and emission characteristics of a hydrogen-diesel dual fuel (HDDF) engine under low load.Compared with traditional HDDF engines, the oxygen-enriched conditions improved the characteristics of low combustion temperature and laminar flame speed under low load, thus reducing ignition delay.However, more NOx was emitted than a diesel-only engine, but this could be reduced through the EGR technique.Since ammonia engines are not widespread, most of the research on ammonia has been on basic combustion characteristics.An experimental and kinetic modeling investigation on the laminar flame propagation of ammonia was conducted by Mei et al. [47] under oxygen-enrichment conditions.It was found the laminar burning velocity increased with the increasing oxygen content in both experimental and numerical studies, which was driven by the enriched concentrations of key radicals H, OH and NH 2 .However, the oxygen enrichment also caused more NOx emissions in turn.Wang [3] investigated the basic characteristics of ammonia fuel in a constant-volume bomb, and the modeling-predicted laminar burning velocities by Cantera code under oxygen enrichment are given in Figure 6.The laminar flame velocity in the ammonia-burning system increased first and then declined as the equivalence ratio changed from 0.6 to 1.5, predicted by varied mechanisms.The greater the oxygen enrichment, the faster the laminar flow flame speed.Meanwhile, there was an interesting phenomenon: the equivalent ratio corresponding to the maximum laminar flame velocity moved towards the lean combustion side with increasing oxygen enrichment Ω.Similar findings were discovered for NOx emissions.As a result, when considering oxygen enrichment to assist ammonia combustion, NOx emissions should be given special attention.
Energies 2023, 16, x FOR PEER REVIEW 9 of 21 et al. [47] under oxygen-enrichment conditions.It was found the laminar burning velocity increased with the increasing oxygen content in both experimental and numerical studies, which was driven by the enriched concentrations of key radicals H, OH and NH2.However, the oxygen enrichment also caused more NOx emissions in turn.Wang [3] investigated the basic characteristics of ammonia fuel in a constant-volume bomb, and the modeling-predicted laminar burning velocities by Cantera code under oxygen enrichment are given in Figure 6.The laminar flame velocity in the ammonia-burning system increased first and then declined as the equivalence ratio changed from 0.6 to 1.5, predicted by varied mechanisms.The greater the oxygen enrichment, the faster the laminar flow flame speed.Meanwhile, there was an interesting phenomenon: the equivalent ratio corresponding to the maximum laminar flame velocity moved towards the lean combustion side with increasing oxygen enrichment Ω.Similar findings were discovered for NOx emissions.As a result, when considering oxygen enrichment to assist ammonia combustion, NOx emissions should be given special attention.

Ammonia-Hydrogen Mixed Combustion
To improve the combustion characteristics of ammonia, it is effective to blend hydrogen with ammonia fuel because hydrogen laminar combustion is faster while having a wider combustion limit compared with ammonia.Differently from dual-fuel combustion or oxygen-enriched combustion, addition of hydrogen derived from partially cracked ammonia typically improves the combustion properties under the conditions of high temperature, high pressure and suitable catalyst.A basic investigation on ammonia decom-

Ammonia-Hydrogen Mixed Combustion
To improve the combustion characteristics of ammonia, it is effective to blend hydrogen with ammonia fuel because hydrogen laminar combustion is faster while having a wider combustion limit compared with ammonia.Differently from dual-fuel combustion or oxygen-enriched combustion, addition of hydrogen derived from partially cracked ammonia typically improves the combustion properties under the conditions of high temperature, high pressure and suitable catalyst.A basic investigation on ammonia decomposition was conducted by Ganley et al. [48], who evaluated the activity of metal catalysts: Ru > Ni > Rh > Co > Ir > Fe > Pt > Cr > Pd > Cu >> Te, Se, Pb.Comptti et al. [49] developed a hydrogen generation system (HGS) for ammonia-hydrogen-fueled internal combustion engines using a commercial ruthenium-based catalyst named ACTA 10010.The HGS heated by exhausted combustion gases performed well in hydrogen production, engine brake thermal efficiency and net heat release, but poorly for NOx emissions.Similar results can be found in the work by Ryu et al. [50], and it is emphasized that only a low ammonia flow rate would improve combustion performance.
Apart from dissociating NH 3 in situ from hydrogen, another method of preparing an NH 3 /H 2 mixture is to inject hydrogen and ammonia into the intake manifold in the gaseous phase separately [51].Zhang et al. [52] conducted a numerical investigation on the effects of hydrogen-rich reformate addition on the combustion and emission characteristics of an ammonia engine.It was found that the in-cylinder pressure and heat release rate increased almost linearly with the increasing reformate blending ratio (R re ) in stoichiometric cases.This is because more H and OH radicals were generated by the reaction H 2 + O→H + OH and the production would promote the consumption of NH 3 .The increase in the combustion temperature resulting from greater R re also reduced the emission of unburned NH 3 and N 2 O, but too high a reforming ratio would appear undesirable, and thus they recommended a ratio of 7.5-10% near stoichiometry.Li et al. [53] investigated the effects of an ammonia-hydrogen mixture on combustion stability in a single-cylinder, four-value optical SI engine.The results showed that compared with a pure ammonia engine, the misfire phenomenon could be avoided by 5% hydrogen addition, 7.5% was the best thermal efficiency, and more than 20% would lead to unstable combustion.This is because a small amount of hydrogen would increase the combustion temperature in the cylinder and accelerate the ammonia oxidation, while further increasing the hydrogen content would cause heat loss, resulting in a decrease in the indicated thermal efficiency.
To understand the effect of cracking ratio on ammonia combustion, Mei et al. [54] performed both experimental and kinetic numerical investigations on laminar flame propagation of partially cracked NH 3 /air mixtures.It was reported the laminar flame speed of the mixture was improved as the cracking ratio increased.The combustion was comparable to methane combustion with a cracking ratio of 40% at atmospheric pressure.Wang [3] investigated the variations in laminar flame speeds with different ammonia oxidation mechanisms at low hydrogen-doping ratios under ambient temperature and pressure conditions.It was found that the laminar flame speed increased significantly with increasing hydrogendoping ratio α in both simulation and available experiments, since more O/H radicals produced by hydrogen accelerated the oxidation reaction of ammonia.Nevertheless, more NO was formed because NH radicals converted directly to NO without HNO oxidation and the conversion of NO to N 2 O would be reduced by higher flame temperature.
[55] investigated the behaviors of an ammonia-fueled engine at different H 2 concentrations, equivalence ratios and boosted pressures.It was found that an engine with high hydrogen concentrations performed well under lean combustion conditions, since the addition of H 2 promoted ignition and combustion stability.Wang et al. [56] studied the combustion characteristics of a NH 3 /H 2 mixture with high hydrogen doping ratio (30%) in a medium-speed marine engine.As the equivalence ratio increased, it failed to ignite unless the initial intake temperature and pressure increased, but this limitation could be overcome when moderate proportion of hydrogen doping was added, for example, the peak flame temperature exceeded 2150 K at a 40% hydrogen doping ratio.However, if more hydrogen is required to achieve the assisted-ignition effect [57], it brings new challenges to ensure safe engine operation, since a large amount of additional hydrogen needs to be stored for fuel supply.In general, dual-fuel compression ignition engine operation with ammonia is dependent on the cetane number of pilot fuel and its injection strategies.The concerns of high unburned ammonia and NOx emissions because of the fuel-bound nitrogen are expected to be mitigated by an aftertreatment system.Thus, ammonia can be positively seen as a feasible solution as an alternative fuel for ICEs, without significant engine refit.

Principle of Plasma-Assisted Combustion
Plasma is the fourth kind of matter distinguished from solid, liquid and gas, consisting of a large number of charged particles and neutral particles, which are electrically neutral.It can be classified into thermal equilibrium plasma and nonequilibrium plasma.Thermal or equilibrium plasmas are characterized by high energy density and equality between the temperature of heavy particles and electrons, while conversely a nonequilibrium plasma characteristic is a lower-pressure plasma with low ion and neutral temperatures, as shown in Figure 7. Nonequilibrium plasma plays a similar role to a catalyst to activate ambient gas molecules at room temperature and pressure through colliding and dissociating chemical bonding with the help of high-energy electrons [58,59].Plasma generation methods commonly used are dielectric barrier discharge (DBD), atmospheric pressure plasma jet (APPJ) and sliding arc discharge plasma (GAD), while pulsed power is the most commonly used excitation power source for atmospheric low-temperature plasma [60].The enhancement of plasma-assisted combustion is achieved mainly through three pathways, as illustrated in Figure 8: thermodynamic effects, kinetic effects and transport [61].Thermodynamically, the increase in overall gas and electron temperatures is ascribed to the energy conversion of the plasma discharge and the exotherm of chemical reactions, thus further accelerating the reactions.Kinetically, a large number of high-energy electrons and active particles involved in plasma not only facilitate chemical reactions due to the reduced activation energy but also introduce new branched reactions in the original system.For transport, plasma discharge will form ionic wind, which can accelerate the mixing process of reaction gases.
As mentioned, plasma-assisted combustion involves complicated plasma dynamics and combustion kinetics, so it is difficult to decouple the plasma effect and thermal effect from the perspective of conventional experiments due to cross-disciplinary complexity.
Although it has been verified that plasma can play a significant role in enhancing combustion, modulating emissions and fuel reforming, the underlying mechanisms are a worthy ongoing topic in different application scenarios.The enhancement of plasma-assisted combustion is achieved mainly through three pathways, as illustrated in Figure 8: thermodynamic effects, kinetic effects and transport [61].Thermodynamically, the increase in overall gas and electron temperatures is ascribed to the energy conversion of the plasma discharge and the exotherm of chemical reactions, thus further accelerating the reactions.Kinetically, a large number of high-energy electrons and active particles involved in plasma not only facilitate chemical reactions due to the reduced activation energy but also introduce new branched reactions in the original system.For transport, plasma discharge will form ionic wind, which can accelerate the mixing process of reaction gases.

PAC Ignition Enhancement
It was found plasma-assisted combustion (PAC) can overcome the flame extinction limit and that PAC ignition enables lower-temperature combustion in contrast to hightemperature ignition, while it must follow the ignition S-curves in conventional ignition process, as shown in Figure 9 [61].For internal combustion engines, plasma generated by different types of discharge such as microwave [62], radio frequency [63], laser ignition [64] and nanosecond pulses (NRP) [65,66], has been used to assist ignition and combustion.Among these, NRP has a significant advantage applied in low-energy ignition.To clarify the cumulative effect of repetitive NRP pulse number on the PAC, Barleon et al. [67] investigated the ignition of a premixed methane-air mixture by NPR in pin-pin configuration.The conclusion was that there was less total energy required as the pulse frequency increased.Compared with traditional spark ignition, Cathey et al. [65] found that it caused shorter ignition delay, higher peak pressure and greater net heat release in a single-cylinder gasoline engine through NRP.In fact, the expected minimum ignition energy is controlled by the minimum flame radius [61], which has a strong relationship to the Lewis number of the mixture, the fuel reactivity (activation energy) and the flame thickness.Therefore, the reduced ignition delay and lowered ignition energy by PCA were to a great extent due to the increase in the diversity of the reaction system (reduced Lewis number) and the decrease in activation energy.
The reduced electric field E/N is the most significant parameter to control the distribution of energy deposited to the different excitation modes and then generate active particles.Research to figure out the best reduced electric field to minimize the ignition time of CH4/Air/He mixture was conducted by Mao et al. [68] using a combination of nanosecond pulses and DC discharges.In their study, a DC electric field of 5 Td minimized the time used for ignition, as shown in Figure 10, and it was revealed that the low-DC electric field promoted the excitation of CH4(v) and O2(v) effectively, as well as O2(a1Δg), which played a positive role in ignition enhancement through pathway flux analysis.As mentioned, plasma-assisted combustion involves complicated plasma dynamics and combustion kinetics, so it is difficult to decouple the plasma effect and thermal effect from the perspective of conventional experiments due to cross-disciplinary complexity.
Although it has been verified that plasma can play a significant role in enhancing combustion, modulating emissions and fuel reforming, the underlying mechanisms are a worthy ongoing topic in different application scenarios.

PAC Ignition Enhancement
It was found plasma-assisted combustion (PAC) can overcome the flame extinction limit and that PAC ignition enables lower-temperature combustion in contrast to hightemperature ignition, while it must follow the ignition S-curves in conventional ignition process, as shown in Figure 9 [61].For internal combustion engines, plasma generated by different types of discharge such as microwave [62], radio frequency [63], laser ignition [64] and nanosecond pulses (NRP) [65,66], has been used to assist ignition and combustion.Among these, NRP has a significant advantage applied in low-energy ignition.To clarify the cumulative effect of repetitive NRP pulse number on the PAC, Barleon et al. [67] investigated the ignition of a premixed methane-air mixture by NPR in pin-pin configuration.The conclusion was that there was less total energy required as the pulse frequency increased.Compared with traditional spark ignition, Cathey et al. [65] found that it caused shorter ignition delay, higher peak pressure and greater net heat release in a single-cylinder gasoline engine through NRP.In fact, the expected minimum ignition energy is controlled by the minimum flame radius [61], which has a strong relationship to the Lewis number of the mixture, the fuel reactivity (activation energy) and the flame thickness.Therefore, the reduced ignition delay and lowered ignition energy by PCA were to a great extent due to the increase in the diversity of the reaction system (reduced Lewis number) and the decrease in activation energy.
The reduced electric field E/N is the most significant parameter to control the distribution of energy deposited to the different excitation modes and then generate active particles.Research to figure out the best reduced electric field to minimize the ignition time of CH 4 /Air/He mixture was conducted by Mao et al. [68] using a combination of nanosecond pulses and DC discharges.In their study, a DC electric field of 5 Td minimized the time used for ignition, as shown in Figure 10, and it was revealed that the low-DC electric field promoted the excitation of CH 4 (v) and O 2 (v) effectively, as well as O 2 (a1∆g), which played a positive role in ignition enhancement through pathway flux analysis.Furthermore, the time scale of plasma kinetics is within the order of nanoseconds, while the combustion kinetics fall in ranges from micro-to milliseconds [61], as illustrated in Figure 11.Therefore, the PAC process is the coupling of long-life plasma species with active radicals from fuel pyrolysis via energy transfer and kinetic interaction.From this point of view, the bridge linking fast plasma kinetics and combustion kinetics is critical to fully explore the PAC principles.This motivates the development an efficient numerical model for multi-timescale PAC.Recently, the zero-dimensional plasma kinetic solver ZDplaskin incorporated with combustion kinetic solver CHEMKIN has been the most widely used numerical tool for PAC.ZDplaskin integrated with Boltzmann equation solver (BOLSIG+) is used to predict time evolution of neural radical and active species.Rate coefficients are formulated based on the incident energy by plasma discharges and collisional cross sections data of electron-associated reaction in LXCat format.Many studies have employed the ZDplaskin-CHEMKIN solver to study flame stability, contemplating the effects of plasma discharges on the flame propagation characteristics and ignition  Furthermore, the time scale of plasma kinetics is within the order of nanoseconds, while the combustion kinetics fall in ranges from micro-to milliseconds [61], as illustrated in Figure 11.Therefore, the PAC process is the coupling of long-life plasma species with active radicals from fuel pyrolysis via energy transfer and kinetic interaction.From this point of view, the bridge linking fast plasma kinetics and combustion kinetics is critical to fully explore the PAC principles.This motivates the development of an efficient numerical model for multi-timescale PAC.Recently, the zero-dimensional plasma kinetic solver ZDplaskin incorporated with combustion kinetic solver CHEMKIN has been the most widely used numerical tool for PAC.ZDplaskin integrated with Boltzmann equation solver (BOLSIG+) is used to predict time evolution of neural radical and active species.Rate coefficients are formulated based on the incident energy by plasma discharges and collisional cross sections data of electron-associated reaction in LXCat format.Many studies have employed the ZDplaskin-CHEMKIN solver to study flame stability, contemplating the effects of plasma discharges on the flame propagation characteristics and ignition Furthermore, the time scale of plasma kinetics is within the order of nanoseconds, while the combustion kinetics fall in ranges from micro-to milliseconds [61], as illustrated in Figure 11.Therefore, the PAC process is the coupling of long-life plasma species with active radicals from fuel pyrolysis via energy transfer and kinetic interaction.From this point of view, the bridge linking fast plasma kinetics and combustion kinetics is critical to fully explore the PAC principles.This motivates the development of an efficient numerical model for multi-timescale PAC.Recently, the zero-dimensional plasma kinetic solver ZDplaskin incorporated with combustion kinetic solver CHEMKIN has been the most widely used numerical tool for PAC.ZDplaskin integrated with Boltzmann equation solver (BOLSIG+) is used to predict time evolution of neural radical and active species.Rate coefficients are formulated based on the incident energy by plasma discharges and collisional cross sections data of electron-associated reaction in LXCat format.Many studies have employed the ZDplaskin-CHEMKIN solver to study flame stability, contemplating the effects of plasma discharges on the flame propagation characteristics and ignition delay times [68,69].However, there remain challenges in developing plasma kinetic mechanisms accommodating low-temperature and high-pressure conditions.At present, 2-D and 3-D numerical tools are not available for PAC modeling.
Energies 2023, 16, x FOR PEER REVIEW 14 of 21 delay times [68,69].However, there remain challenges in developing plasma kinetic mechanisms accommodating low-temperature and high-pressure conditions.At present, 2-D and 3-D numerical tools are not available for PAC modeling.The dilution of different types of inert gases also affects electron energy distribution function in plasma, thus influencing the mixture combustion reactivity.A numerical investigation on the effects of methane pyrolysis with different diluents (N2/Ar/He) was conducted by Mao et al. [70] using the zero-dimensional solver ZDPlasKin coupled with CHEMKIN.The results indicated that the quenching of electronic excited states and relaxation of the vibrational state contributed greatly to the pyrolysis of CH4, for which the addition of N2 as diluent performed best.As a result, it is concluded the excited states of equilibrium gases play a dominant role in physicochemical mechanisms and fuel oxidation efficiency through collisional quenching reactions.Similar findings were reported by Snoeckx et al. [71].
The commercial applications of plasma on ICEs are still immature, but many investigations are being carried out.Hwang et al. [72] developed a new microwave-assisted plasma ignition system and applied it to a direct injection gasoline engine.The new ignition system has a shorter ignition delay and a more advanced combustion stage than conventional spark ignition systems.It also broadened the thin combustion limit since it can form a larger fire nucleus by providing abundant reactive radicals over a larger area.A laser-induced plasma (LIP) ignition system has been evaluated to assist the ignition of a diesel-gasoline mixture in a CI engine by José et al. [73].It was reported that the LIP ignition system can effectively solve the problem of automatic ignition of mixed fuels under different operations, and in addition, the variable height ignition technology improved combustion efficiency and reduced hydrocarbon emissions.L. et al. [74] investigated the ignition behavior of a methane-air mixture in an optically equipped setup consisting of a double chamber under repeated pulse discharge (NRPD).It showed that NRPD has a higher ignition success rate and average flame front propagation compared to standard inductive ignition.It can achieve lean combustion by adjusting the energy and number of pulses, but it did not work after inflammation.

Plasma-Assisted Ammonia Ignition
As mentioned above, addition of hydrogen derived from partially cracked ammonia typically improves the combustion properties.Direct decomposition of ammonia to The dilution of different types of inert gases also affects electron energy distribution function in plasma, thus influencing the mixture combustion reactivity.A numerical investigation on the effects of methane pyrolysis with different diluents (N 2 /Ar/He) was conducted by Mao et al. [70] using the zero-dimensional solver ZDPlasKin coupled with CHEMKIN.The results indicated that the quenching of electronic excited states and relaxation of the vibrational state contributed greatly to the pyrolysis of CH 4 , for which the addition of N 2 as diluent performed best.As a result, it is concluded the excited states of equilibrium gases play a dominant role in physicochemical mechanisms and fuel oxidation efficiency through collisional quenching reactions.Similar findings were reported by Snoeckx et al. [71].
The commercial applications of plasma on ICEs are still immature, but many investigations are being carried out.Hwang et al. [72] developed a new microwave-assisted plasma ignition system and applied it to a direct injection gasoline engine.The new ignition system has a shorter ignition delay and a more advanced combustion stage than conventional spark ignition systems.It also broadened the thin combustion limit since it can form a larger fire nucleus by providing abundant reactive radicals over a larger area.A laser-induced plasma (LIP) ignition system has been evaluated to assist the ignition of a diesel-gasoline mixture in a CI engine by José et al. [73].It was reported that the LIP ignition system can effectively solve the problem of automatic ignition of mixed fuels under different operations, and in addition, the variable height ignition technology improved combustion efficiency and reduced hydrocarbon emissions.L. et al. [74] investigated the ignition behavior of a methane-air mixture in an optically equipped setup consisting of a double chamber under repeated pulse discharge (NRPD).It showed that NRPD has a higher ignition success rate and average flame front propagation compared to standard inductive ignition.It can achieve lean combustion by adjusting the energy and number of pulses, but it did not work after inflammation.

Plasma-Assisted Ammonia Ignition
As mentioned above, addition of hydrogen derived from partially cracked ammonia typically improves the combustion properties.Direct decomposition of ammonia to hydrogen is a typical endothermic reaction with an enthalpy change of 91.2 kJ/mol.The decomposition of ammonia is able to be achieved at high temperature in the presence Energies 2023, 16, 6304 14 of 20 of a classic selective catalyst.Nevertheless, plasma catalysis is a promising approach to decompose ammonia at low temperature (<450 • C), but with a relatively high conversion rate.The synergistic effect of plasma and catalyst refers to the truth that it affects the catalytic process more than the sum of plasma and catalyst alone [75].Although plasma catalytic ammonia synthesis is well understood, plasma catalytic ammonia decomposition still needs further study, and there is no commercial application available so far.Research on plasma catalytic ammonia decomposition has been centered on experiments, most of which have focused on designing high-efficient catalyst to improve hydrogen conversion.Wang et al. [76] measured the effect of DBD plasma synergism with Fe-based catalysts on ammonia decomposition efficiency through optical diagnostics and isotope scanning techniques.It was revealed the excited-state NH 3 * and NH* played an important role in N atoms desorption, thus influencing conversion efficiency as the reaction temperature reduced by 100-140 • C. In their subsequent study [77], it was found the plasma promoted the chemisorption of NH 3 *, leading to increased conversion by nearly 40% at 550 C. The underlying physicochemical mechanisms of plasma catalytic kinetics should be further explored in depth to facilitate its practical usage in on-site hydrogen generation.
A number of simulation studies were carried out by using the kinetic model under wide conditions of temperatures ranging from 600 to 1500 K, and the characteristics of plasma-assisted ammonia combustion were predicted with varying pulsation frequencies and pulse numbers.Faingold and Lefkowitz [78] employed ZDplaskin-CHEMKIN with an assembled kinetic model for the oxidation of ammonia/oxygen/helium to perform pulse repetition frequencies (PRFs) on ignition delay times (IDTs).It was found IDTs could be reduced by 40-60%, as shown in Figure 12, under a moderate number of pulses.Higher PRFs promote an expanding radical pool, whereas lower PRFs favor the radical recombination between pulses.hydrogen is a typical endothermic reaction with an enthalpy change of 91.2 kJ/mol.The decomposition of ammonia is able to be achieved at high temperature in the presence of a classic selective catalyst.Nevertheless, plasma catalysis is a promising approach to decompose ammonia at low temperature (<450 °C), but with a relatively high conversion rate.The synergistic effect of plasma and catalyst refers to the truth that it affects the catalytic process more than the sum of plasma and catalyst alone [75].Although plasma catalytic ammonia synthesis is well understood, plasma catalytic ammonia decomposition still needs further study, and there is no commercial application available so far.Research on plasma catalytic ammonia decomposition has been centered on experiments, most of which have focused on designing high-efficient catalyst to improve hydrogen conversion.Wang et al. [76] measured the effect of DBD plasma synergism with Fe-based catalysts on ammonia decomposition efficiency through optical diagnostics and isotope scanning techniques.It was revealed the excited-state NH3* and NH* played an important role in N atoms desorption, thus influencing conversion efficiency as the reaction temperature reduced by 100-140 °C.In their subsequent study [77], it was found the plasma promoted the chemisorption of NH3*, leading to increased conversion by nearly 40% at 550 °C.The underlying physicochemical mechanisms of plasma catalytic kinetics should be further explored in depth to facilitate its practical usage in on-site hydrogen generation.
A number of simulation studies were carried out by using the kinetic model under wide conditions of temperatures ranging from 600 to 1500 K, and the characteristics of plasma-assisted ammonia combustion were predicted with varying pulsation frequencies and pulse numbers.Faingold and Lefkowitz [78] employed ZDplaskin-CHEMKIN with an assembled kinetic model for the oxidation of ammonia/oxygen/helium to perform pulse repetition frequencies (PRFs) on ignition delay times (IDTs).It was found IDTs could be reduced by 40-60%, as shown in Figure 12, under a moderate number of pulses.Higher PRFs promote an expanding radical pool, whereas lower PRFs favor the radical recombination between pulses.A series of experiments on plasma-assisted ammonia combustion were carried out because of the great advantages of plasma-assisted combustion as mentioned before.Lin et al. [79] studied the performance of plasma-assisted combustion via a new gliding arc plasma (GAP) generator combined with a cyclonic burner.The operation map of ammonia combustion flames with the GAP on or off is displayed in Figure 13, in which the ammonia combustion limitation was widened under both air and ammonia GAP.However, if the equivalence ratio exceeded 2.2 under a high air flow rate, the ammonia A series of experiments on plasma-assisted ammonia combustion were carried out because of the great advantages of plasma-assisted combustion as mentioned before.Lin et al. [79] studied the performance of plasma-assisted combustion via a new gliding arc plasma (GAP) generator combined with a cyclonic burner.The operation map of ammonia combustion flames with the GAP on or off is displayed in Figure 13, in which the ammonia combustion limitation was widened under both air and ammonia GAP.However, if the equivalence ratio exceeded 2.2 under a high air flow rate, the ammonia combustion flame would become unstable.Furthermore, the physicochemical mechanisms of discharged air and ammonia worked differently.The particles (OH*, H* and O*) played a dominant role in the dehydrogenation reaction of ammonia through kinetic mechanisms for the former (air medium) while the latter (ammonia medium) improved the combustion limit by the hydrogen produced from ammonia directly.A similar investigation on ammonia combustion was conducted by Choe et al. [80] using a nanosecond high-pressure pulse generator.It was concluded that plasma was able to extend the lean blowoff limits of ammonia flames and reduce NOx emissions simultaneously, but further research needs to be focused on the interaction between plasma dynamics and combustion kinetics.

Conclusions
The outlook of independence from conventional fossil fuels is decarbonization in the automotive, marine and power generation sectors.Towards this goal, running with carbon-free fuel such as ammonia in ICEs is drawing more attention in research activities.Currently, ammonia can be ignited with diesel or any other high-reactivity fuel in dual- Plasma-assisted ammonia combustion has great capacity for shortening ignition delay timing and extending combustible ranges while reducing NOx emissions.It will definitely promote the use of ammonia in engines.This has attracted lots of attention from scholars dedicated to figuring out the underlying physicochemical mechanisms via theoretical analysis and numerical simulation.Taneja et al. [81] found that PAC achieved the fastest ignition in a lean fuel mixture because of the accumulation of OH radicals through the reactivity-inhibiting reactions between plasma pulses, while an inversely proportional impact on ignition delay times was exhibited on plasma pulse frequency and energy density deposited.Moreover, the reforming of ammonia to nitrogen resulted in lower production of NOx with plasma.Another similar investigation also discovered PAC changed the conventional ignition and extinction characteristics, and the S-shaped curves were replaced by the monotonic and stretched ignition curves, which made low-temperature ignition possible [78].Shahsavar et al. [82] figured out the most effective range of a reduced electric field on ammonia equivalent ratio combustion: 250-400 Td.This greatly promoted ignition time by increasing the reduced electric field in the early stage.With a more reduced electric field, a large fraction of the energy in the plasma system would be utilized in the ionization reactions of the diluent, thus neutralizing the effective excitations of fuel and oxidizer species, as shown in Figure 14.

Conclusions
The outlook of independence from conventional fossil fuels is decarbonization in the automotive, marine and power generation sectors.Towards this goal, running with carbon-free fuel such as ammonia in ICEs is drawing more attention in research activities.Currently, ammonia can be ignited with diesel or any other high-reactivity fuel in dualfuel mode, especially in marine engines and heavy-duty engines.Moreover, the addition of hydrogen is able to be derived from partially cracked ammonia with suitable catalyst in situ.However, the optimal hydrogen ratio in ammonia-hydrogen mixed combustion should not exceed 10%, considering thermal efficiency and combustion stability.A promising technology of plasma-assisted combustion to overcome the bottleneck of ammonia Finally, when projecting the adoption of plasma as an encouraging approach to boost ammonia decomposition and ignition, this review summarizes the latest innovations in the field of plasma catalysis and PAC, including progress in both numerical models and experimental studies.Great interest in improving ignition delay timing, increasing flame speed, and extending flammability limits while reducing NOx emissions will promote fast application in practical ICEs.

Conclusions
The outlook of independence from conventional fossil fuels is decarbonization in the automotive, marine and power generation sectors.Towards this goal, running with carbon-free fuel such as ammonia in ICEs is drawing more attention in research activities.Currently, ammonia can be ignited with diesel or any other high-reactivity fuel in dual-fuel mode, especially in marine engines and heavy-duty engines.Moreover, the addition of hydrogen is able to be derived from partially cracked ammonia with suitable catalyst in situ.However, the optimal hydrogen ratio in ammonia-hydrogen mixed combustion should not exceed 10%, considering thermal efficiency and combustion stability.A promising technology of plasma-assisted combustion to overcome the bottleneck of ammonia limits has attracted positive interest.The fundamental mechanisms of possible technical advances emerging in assisted combustion of ammonia are reviewed in this study.
(1) The laminar burning velocity of ammonia combustion increased with the increasing oxygen content, driven by the enriched concentrations of key radicals H, OH and NH 2 .The maximum laminar flame velocity corresponds to equivalent ratio combustion, but the maximum NOx emissions moved towards lean combustion with increased oxygen enrichment.
(2) Investigations of ammonia-fueled engines have been widely carried out with varied additions of hydrogen, but modulations of inlet temperature and pressure are still necessary, especially for lean combustion.If more hydrogen is needed, it must be a new challenge to ensure operation safety with hydrogen supply.
(3) Plasma-assisted combustion enables lower-temperature combustion and has ability to overcome the flame extinction limit while reducing NOx emissions, due to the increase in diversity via active particles in the reaction system and the decrease in activation energy.The underlying physicochemical mechanisms of plasma-assisted ammonia combustion are rarely reported.The accumulation of OH radicals through reactivity-inhibiting reactions in lean fuel combustion could be stimulated through thermodynamic and kinetic effects via proper plasma generator configuration.

Figure 1 .
Figure 1.Simplified NH 3 production progress with CH 4 as the main raw material [19].

Figure 3 .
Figure 3. Combustion durations versus ignition timing at different ammonia levels [29].(a) duration of CA0-10 for varied addition of ammonia (b) duration of CA10-90 for varied addition of ammonia.

Figure 3 .
Figure 3. Combustion durations versus ignition timing at different ammonia levels [29].(a) duration of CA0-10 for varied addition of ammonia (b) duration of CA10-90 for varied addition of ammonia.

Figure 3 .
Figure 3. Combustion durations versus ignition timing at different ammonia levels [29].(a) duration of CA0-10 for varied addition of ammonia (b) duration of CA10-90 for varied addition of ammonia.

21 Figure 5 .
Figure 5. Temperature distribution and integrated production rate such as NO varies with α, φ = 0.75, CR = 10 [28].4.Current Technologies of Ammonia-Fueled Engines4.1.Oxygen-Enriched CombustionIt has been confirmed that oxygen-enriched combustion has the ability to lower fuel ignition points, speed up reactions, widen combustion limits and raise flame temperature.

Figure 6 .
Figure 6.Comparison of predicted laminar flame velocity of ammonia with low oxygen-rich concentration [3].

Figure 6 .
Figure 6.Comparison of predicted laminar flame velocity of ammonia with low oxygen-rich concentration [3].

Energies 2023 ,
16,  x FOR PEER REVIEW 11 of 21 generation methods commonly used are dielectric barrier discharge (DBD), atmospheric pressure plasma jet (APPJ) and sliding arc discharge plasma (GAD), while pulsed power is the most commonly used excitation power source for atmospheric low-temperature plasma[60].

Figure 9 .
Figure 9. Schematic diagram of the plasma-assisted transition from the classical ignition and extinction S-curve (blue solid line) to the monotonic stretching S-curve (red dashed line) [61].

Figure 10 .
Figure 10.Time evolution of O2(a1Δg) for NSD and mixed discharges with different DC field strengths at 900 K [68].

Figure 9 . 21 Figure 9 .
Figure 9. Schematic diagram of the plasma-assisted transition from the classical ignition and extinction S-curve (blue solid line) to the monotonic stretching S-curve (red dashed line) [61].

Figure 10 .
Figure 10.Time evolution of O2(a1Δg) for NSD and mixed discharges with different DC field strengths at 900 K [68].

Figure 10 .
Figure 10.Time evolution of O 2 (a1∆g) for NSD and mixed discharges with different DC field strengths at 900 K [68].

Energies 2023 , 21 Figure 13 .
Figure 13.An operation map of ammonia combustion flames under the following conditions: (a) air GAP working on/off and (b) ammonia GAP working on.

Figure 13 .
Figure 13.An operation map of ammonia combustion flames under the following conditions: (a) air GAP working on/off and (b) ammonia GAP working on.

Figure 13 .
Figure 13.An operation map of ammonia combustion flames under the following conditions: (a) air GAP working on/off and (b) ammonia GAP working on.

Figure 14 .
Figure 14.Effects of reduced electric field on ignition delay times and laminar flame velocities for plasma-assisted NH 3 /O 2 /N 2 combustion [82].