Ammonia as Fuel for Future Diesel Engines

Abstract

Ammonia (NH₃) is one of the important ways for diesel engines to achieve carbon neutrality. Ammonia’s energy density by volume is nearly double that of liquid hydrogen, making it easier to ship and distribute. Ammonia has a well-developed infrastructure and can also be used as a hydrogen energy carrier. However, it was discovered that using pure ammonia as fuel was impracticable, prompting researchers to create concepts for dual-fuel systems or innovative combustion techniques. Therefore, a detailed literature review was conducted on applying ammonia in diesel engines. Firstly, the development of ammonia as a fuel, green ammonia production, ammonia’s physicochemical characteristics, and challenges were discussed. Then, using ammonia as fuel in a dual-fuel compression ignition engine was emphasized, with secondary fuels such as diesel, dimethyl ether, hydrogen, and other alternative fuels. Advanced injection strategies help improve engine combustion performance and reduce emissions. Due to the low flame velocity, long quenching distance, and fuel-bound nitrogen of ammonia, there are high levels of NO_x and unburned NH₃ in the exhaust, which makes it necessary to use after-treatment systems downstream. The NH₃-H₂ homogeneous charge compression ignition mode and ammonia cracking are also presented.

1. Introduction

With the widespread usage of fossil fuels, environmental issues such as the greenhouse effect have become more serious. Aiming to achieve net zero greenhouse gas emissions in the second half of this century, the Paris Agreement was ratified by over 200 parties to the United Nations Framework Convention on Climate Change in 2015 [1]. In order to achieve the above targets, it is essential to replace a significant percentage of fossil fuels with renewable energy sources. However, the majority of renewable energy sources, like wind, wave, tidal, and solar, produce energy intermittently. As a result, storing the energy in batteries or chemical form is important to mitigate the consequences of fluctuations in energy output [2]. Chemical storage is more economical in comparison to batteries. Chemical storage systems, which have a lower levelized cost of energy storage and allow for the storage of energy produced from renewable sources by converting it into fuel (power-to-fuel), can be used to store energy for longer periods of time and in larger amounts [3]. Therefore, hydrogen and ammonia are proposed as alternative energy sources available in chemical form.

As the main source of greenhouse gas (GHG) in transportation, internal combustion engines face a major challenge and opportunity [4]. The most practical way to minimize GHG emissions is to use carbon-free alternative fuels. Recently, green hydrogen produced by water electrolysis (using electricity from renewable energy sources) has drawn a lot of interest as the future fuel, but its implementation has been constrained by problems with hydrogen storage and delivery [5]. Ammonia has been highlighted as an energy carrier for the green energy (zero-emission) cycle because of its ability to act as a hydrogen energy carrier for the storage and transit of green hydrogen [6]. The findings of recent research, which showed that GHG emissions from ammonia-fueled engines are less than one-third of those from traditional engines fueled with fossil fuels, demonstrate the potential of ammonia to serve as a power-to-fuel for sustainable energy future [7].

Compression ignition (CI) engines have a higher installed capacity and higher thermal efficiency than spark ignition (SI) engines in the transportation and power generation sectors [8]. The shipping industry uses over 330 million metric tons of fossil fuels a year, which increases the amount of hazardous exhaust emissions that contaminate the environment [9]. The International Maritime Organization’s implementation of rigorous emission limits for the shipping industry’s decarbonization prompted a search for alternative fuels, and ammonia caught the attention of researchers as a carbon-free fuel [10]. This work aims to provide a comprehensive analysis of historical and present research activity on ammonia applications in CI engines. The chapter’s organization is as follows: Section 1 provides an overview of the evolution of ammonia as a fuel, the production of green ammonia, the physical and chemical characteristics of ammonia, and the difficulties when ammonia is used as a fuel in CI engines. Section 2 contains a detailed literature review of ammonia-fueled CI engines’ performace, with Sections 2.1, 2.2, 2.3, and 2.4 covering the single-fuel combustion of ammonia and the dual-fuel combustion of ammonia with diesel, dimethyl ether (DME), and other fuels, respectively. Section 3 introduces advanced combustion technologies for ammonia CI engines, including homogeneous charge compression ignition (HCCI) (3.1) and ammonia cracking for hydrogen production (3.2). Section 4 lists the after-treatment measures for the ammonia CI engine’s emissions. Section 5 provides a summary of the literature review.

1.1 Development of ammonia fuel

As illustrated in Figure 1, the use of ammonia as a fuel for transportation dates back to the early 1800s. Small locomotives and trams were the primary use for ammonia during this period. Privately owned ammonia-powered vehicles first appeared in the early to mid-1900s, going through propulsion technology with NASA’s X-15 program in 1965. The most recent advancements in ammonia-fueled vehicles happened in the 2010s, with the most current usage in a sports car [11].

Figure 1.

The development of ammonia as a fuel for internal combustion engines, reprinted from [11].

The study of ammonia as a fuel for internal combustion engines was undertaken in two stages, each with a distinct goal. The initial phase of research took place during the 1960s and 1970s, following the Second World War, with the goal of addressing the oil and logistical crises. The major goal of the second phase of ammonia fuel research, which began in the new century, was to reduce greenhouse gas emissions through the creation of carbon-free fuels [12].

1.2 Production process of green ammonia

Ammonia production may be classified into three categories based on its manufacturing process: brown ammonia, produced exclusively from fossil fuels, which is the highest carbon-emitting process; blue ammonia, low-carbon ammonia whose production is still fossil fuel-based, but carbon capture and storage technology is added to the process; and green ammonia, carbon-free ammonia produced exclusively via sustainable electricity, water, and air [11]. Green ammonia production is possible by combining a standard ammonia synthesis loop with electrolysis-based hydrogen, as shown in Figure 2. The electricity used throughout the entire process is generated from renewable energy sources such as wind and solar energy.

Figure 2.

Schematic for green ammonia synthesis combined with electrolysis-based hydrogen production, reprinted from [13].

Green ammonia is produced by the Haber-Bosch process, where hydrogen (from water electrolysis) is combined with nitrogen (from the air separation unit) at high temperature (400–600°C) and pressure (200–400 bar) [14]. The reaction process for ammonia synthesis is shown in Eq. (1):

$$N_2+3H_2\to 2{\mathit{NH}}_3\left(∆H=-92.4\mathit{kJ}/\mathit{mol}\right)$$E1

Ammonia is an important hydrogen energy carrier in the development of green ammonia and green hydrogen production. Major countries have proclaimed policies and incentives for the development of cost-effective and creative technologies for green ammonia production, recognizing the ease of storing and delivering hydrogen energy in the form of green ammonia using their well-established infrastructure. Since 2014, Japan has launched the Energy Carriers technology development consortium, which is supported by the Cross-ministerial Strategic Innovation Promotion Program, emphasizing ammonia’s central role in the hydrogen economy as a carrying energy vector. The first small-scale green ammonia concept plant consists of a 30 kW electrochemical reactor created in Oxford, UK. This machine produces roughly 30 kg of green ammonia per day as a consequence of a collaboration between Siemens and the Universities of Oxford and Cardiff [15]. In addition to Siemens, other large corporations have already established research and development guidelines for the synthesis of carbon-free ammonia supported by renewable energy sources for use in transportation, chemical fertilization, and large-scale power production. These corporations include MAN Energy Solutions [16] and ThyssenKrupp [17].

1.3 Physical and chemical properties of ammonia

The complete combustion reaction of ammonia is shown in Eq. (2)

$${\mathit{NH}}_3+\frac{3}{4}\left(O_2+3.76N_2\right)\to \frac{3}{2}{H}_{2}O+3.32N_2$$E2

There are no emissions of carbon dioxide (CO₂), carbon monoxide (CO), hydrocarbon (HC), or soot from ammonia combustion since it does not contain any carbon atoms. Table 1 compares the thermodynamic characteristics of ammonia and other conventional or alternative fuels in detail. Ammonia is a colorless gas with a strong odor. Its molecular structure consists of one nitrogen atom and three hydrogen atoms. It is lighter than air and alkaline in nature. Ammonia may be erosive to some materials. Ammonia has a larger potential as an energy carrier since it can liquefy at a temperature of 33.34°C under atmospheric pressure, while hydrogen must liquefy at a very low temperature of 252.7°C. Another benefit of using ammonia as fuel is that it can be transported safely in huge amounts because of well-established documented protocols and widespread infrastructure for pipes, roads, and rails. Additionally, ammonia is less dangerous in unintentional fires or explosions than other fuels due to its low reactivity. However, ammonia has certain limitations for engine applications, such as high autoignition temperature, low flame propagation speed, and narrow flammability limits, which make it difficult to utilize as a single fuel in engines. Ammonia’s great latent heat of vaporization restricts its use as a liquid fuel since it requires more energy to vaporize, lowering the in-cylinder temperature and affecting the engine combustion characteristics.

Property	Units	Ammonia	Hydrogen	Methane	Gasoline	Diesel
Density at 1 bar, 25°C	kg/m^3	0.718	0.0837	0.667	736	849
Lower heating value	MJ/kg	18.8	120	50	44.5	45
Latent heat of vaporization	kJ/kg	1370	455	511	348.7	232.4
Boiling point	°C	−33.34	−252.7	−161.5	35–200	282–338
Specific heat capacity Cp	kJ/(kg K)	2.19	14.30	2.483	2.22	1.75
Volumetric energy density at 1 bar, 25°C	GJ/m3	11.3	4.7	9.35	33	36.4
Octane number (RON)		130	>100	120	90–98	8–15
Autoignition temperature	°C	657	500–577	586	230	254–285
Laminar flame speed	cm/s	7	351	38	58	86
Flammability limit (φ)		0.63–1.4	0.1–7.1	0.5–1.7	0.55–4.24	0.8–6.5
Stoichiometric air-fuel ratio by mass		6.05	34.6	17.3	15	14.5
Adiabatic flame temperature	°C	1800	2110	1950	2138	2300

Table 1.

Physical and chemical properties of ammonia and other common fuels [18].

1.4 Challenges of ammonia combustion in CI engines

The physicochemical properties of ammonia determine the following challenges for its application in engines:

• Security and compatibility

• Low reactivity

• High-fuel nitrogen oxide (NO_x) emissions

Both gaseous and liquid ammonia are dangerous to human health. Ammonia inhalation or direct contact causes lung infections, eye discomfort, and skin damage [19]. Considering ammonia is poisonous, it is important to eliminate leaks in the combustion system and remove unburned ammonia from the exhaust gases after combustion. Corrosion is caused when ammonia exposes to metals, including copper, zinc, aluminum, and their alloys. Steel has been used to store ammonia as a replacement. Ammonia can be used as a fuel in a typical internal combustion engine without significantly changing the engine’s geometrical characteristics [20]. The ammonia fuel container and supply system are the main alterations that most typically increase the total area required and weight of the engine system.

To address the inherent drawbacks of the ammonia oxidation process, a variety of highly reactive fuels, such as diesel, biodiesel, hydrogen, and DME, are frequently chosen for co-combustion with ammonia in CI engines. On the other hand, the strategy of combining NH₃ with other fuels as combustion boosters would significantly lessen the requirement for dramatic engine changes [21]. To give maximum power and minimal emissions for each unique fuel, the best blend of ammonia, fuel, and air must be devised [12]. Related studies have succeeded in demonstrating effective operation using NH₃ with no or little design modifications while often deteriorating NO_x emissions. Because the presence of fuel-bound nitrogen in ammonia causes massive NO_x generation during the pyrolysis process in the combustion zone, a better knowledge of NO_x chemistry during ammonia combustion is essential for the development of effective abatement strategies and technologies. Several methods have been presented to minimize NO_x emissions in the combustion system, including moderate or intense low-oxygen dilution combustion, two-stage rich-lean combustion, selective catalytic reduction, or steam addition [22].

2. Ammonia-fueled CI engines

The results of the early-year study on ammonia were unsatisfactory for CI engines due to its high autoignition temperature, low flame speed, narrow flammability limits, and high heat of vaporization [23]. Only the engines designed with extremely high compression ratios (CR) ranging from 35:1 to 100:1 showed successful ammonia operation under CI conditions [24]. Higher CRs are unfeasible in conventional engines since the allowable CR in real-world applications ranges from 12:1 to 24:1 [25]. Therefore, the failed surgery during the first phase might explain the long-term blank observed in the literature. During the second stage, the research aims are primarily focused on reducing greenhouse emissions by partially substituting diesel with ammonia in a dual-fuel operation [26]. Combustion of ammonia in dual-fuel mode with a secondary fuel as a combustion promoter would be feasible for ammonia combustion in CI engines [27].

2.1 Ammonia as a single fuel for CI engines

Gray et al. [28] could only perform ammonia CI combustion at a condition with a CR of 35:1 and an intake gas temperature over 150°C. High-temperature glow coils were proven to be superior ignition sources. The fuel injection system was changed in Ref. [29] to allow for liquid ammonia injection. In a recent study, Lee and Song proposed a new combustion strategy for CI engines fueled with pure ammonia, where a small amount of ammonia (corresponding to equivalence ratios of 0.1–0.3) was pilot injected during the intake process to form a homogeneous lean mixture with air and undergoes autoignition during the compression stroke [30]. This will raise the pressure and temperature within the cylinder to the point where the primary ammonia spray injection will ignite. The specific combustion strategy is shown in Figure 3. A parametric analysis was carried out with the assistance of simulations to examine the engine performance using the suggested combustion approach. Despite the intriguing approach, the authors emphasize that additional work is required. However, the adoption of pure ammonia in CI engines is difficult due to the extraordinarily high CRs required.

Figure 3.

Schematic describing the combustion strategy for pure ammonia combustion, reprinted from [30].

2.2 Ammonia-diesel dual-fuel operation

There are two strategies for ammonia-diesel dual-fuel operation: low-pressure injection dual-fuel (LPDF) mode and high-pressure injection dual-fuel (HPDF) mode. In LPDF mode, ammonia and air are mixed during the intake stroke, and diesel is typically injected into the combustion chamber at the end of the compression stroke to induce the combustion process. According to the experience of LPDF engines using methane as the main fuel, the valve overlap and quenching effects will result in methane slipping into the exhaust gas [31]. The use of ammonia as the main fuel can also result in corresponding situations. For the LPDF mode, the maximum ammonia ratio of roughly 80% by energy is advised, and increasing the diesel replacement ratio will induce the risk of misfire [32].

In comparison to LPDF, the high-pressure direct injection of liquid ammonia not only delivers enough ammonia into a cylinder in less time but also has no volumetric efficiency loss. For the HPDF engines, the high-reactivity pilot diesel fuel is injected into the combustion chamber at the end of the compression stroke and acts as an ignition source for the subsequently injected low-reactivity main fuel of ammonia. Similar to diesel engines, the in-cylinder combustion process is controlled by fuel-air mixing in the HPDF mode. The high CR can be used on the HPDF engine to increase the thermal efficiency without knocking. Furthermore, the diffusive flame combustion process is complete and stable, which reduces unburned fuel slip and broadens the working condition, with the potential to attain a 97% diesel replacement ratio [32]. Figure 4 shows a schematic of engine bench setup with an ammonia-diesel dual direct injection system.

Figure 4.

Schematic of engine bench setup with an ammonia-diesel dual direct injection system, reprinted from [33].

2.2.1 Low-pressure injection dual-fuel mode

Research into the LPDF mode first began in the 1960s. The diesel pilot injection and gaseous ammonia intake port injection were used in a two-cylinder engine with a CR of 18.6:1 [29]. Pearsall performed experiments to examine the effects of changing the amount of diesel delivered to engine in-cylinder while keeping all other factors constant. In comparison to the case where the engine was operating at richer conditions (equivalence ratio = 0.85), it was discovered that when the engine was operating at relatively lean conditions (equivalence ratio = 0.64), increasing the amount of diesel produced a more pronounced increase in power output and decreased brake specific fuel consumption. According to the research [29], a naturally aspirated ammonia-only SI engine could easily produce as much power as a supercharged diesel-ammonia engine. Bro and Pedersen [34] examined the feasibility of premixed ammonia gas compared to methanol, ethanol, and methane in a dual-fuel CI engine in 1977. According to Figure 5, ammonia is the least preferable alternative fuel for dual-fuel combustion in CI engines because of its slow burning velocity, high ignition delay time, and unburned ammonia. Following this study, research into ammonia-fueled CI engines was suspended for a period of time.

Figure 5.

Results of comparative study on alternate fuels for diesel dual-fuel combustion at 1500 rpm and 1.6 kW, reprinted from [12].

With increasing attention to GHG emissions, research on the combustion and emission characteristics of LPDF engines has been revived in recent years. In 2008, Reiter and Kong demonstrated the performance of ammonia-powered CI engines in dual-fuel mode [20]. The engine air intake system was injected with ammonia through stainless steel tubing, and the diesel injection system remained unmodified. They explored the influence of changing the ammonia replacement (energy share ratios) on combustion performance and emission characteristics. The engine successfully ran at a maximum energy share of 95%. For ammonia ratios ranging from 40 to 80%, reasonable fuel economy with a combustion efficiency of roughly 95% was obtained, and the brake thermal efficiency was around 33–38%. When diesel was substituted with ammonia in the fuel blend, the combustion temperature was reduced, resulting in higher HC and CO emissions [35]. Fuel-rich zones inside engine cylinders were the principal sources of soot emission in diesel engines, and replacing diesel with ammonia significantly (>40%) reduced soot emission. The in-cylinder pressure and heat release rate for constant power output conditions of the engine fueled with 60% diesel +40% ammonia (the start of injection (SOI) was 37°CA before top dead center (BTDC)) and 40% diesel +60% ammonia (35°CA BTDC SOI) are shown in Figure 6, where the effects of fuel composition on combustion phasing and peak pressure can be seen in detail. Compared to pure diesel combustion, the addition of ammonia showed a significant increase in ignition delay time. In the case of 60% diesel +40% ammonia combustion, premixed and diffusion combustion phases in traditional diesel combustion were kept, as evident from the history of heat release rate. When the ammonia energy share increased to 60%, the longer ignition delay time resulted in complete mixing and just one peak in the heat release rate.

Figure 6.

In-cylinder pressure and heat release rate of dual-fuel engine fueled with (a) 60% diesel +40% ammonia and (b) 40% diesel +60% ammonia at constant power output (40 kW at 1000 rpm) conditions, reprinted from [35].

In the past few years, researchers have applied advanced diesel injection systems with pilot and postinjections to manage combustion temperature and harmful emissions [36]. In Ref. [37], numerical simulations were carried out to study the effectiveness of diesel, diesel-kerosene, and kerosene pilot injections in an ammonia-fumigated CI engine. For ammonia energy-sharing ratios exceeding 60%, CO and CO₂ emissions were significantly decreased, while NO_x emissions were significantly increased. Yousefi et al. evaluated the ammonia/diesel dual-fuel mode on a heavy-duty diesel engine both experimentally and numerically [38]. A significant amount of unburned ammonia was produced from the poor flame propagation of the premixed ammonia-air mixture. Due to excess NH₃ inside the combustion chamber being used to consume nitric oxide (NO) rather than produce NO, NO_x emissions may be reduced by 58.8% when the ammonia energy share ratio increases from 0 to 40%.

2.2.2 High-pressure injection dual-fuel mode

Despite some experimental and numerical studies on the ammonia/diesel LPDF engines, research on the ammonia/diesel HPDF mode has not been widely documented [32]. For analyzing the spray combustion processes and emissions of diesel engines, three-dimensional computational fluid dynamics simulation has gained acceptance as a trustworthy tool [39]. Researchers from the Technical University of Munich used numerical simulations to explore the application of ammonia in the HPDF engines [40, 41]. After the top dead center (ATDC), the pilot diesel and liquid ammonia were introduced into the cylinder at −2.5 and 1°CA, respectively. The fuel-air mixing rate determines the heat release rate for the HPDF mode, and liquid ammonia has the capacity to supply more than 95% of the total injected energy. This paper gave no information on emissions or engine performance but focused on cylinder pressure and heat release rate.

Accurate spray modeling near the TDC is necessary for a good simulation of the HPDF engines. Li et al. [32] used a constant-volume combustion chamber to measure the spray characteristics of liquid ammonia. The numerical model for the ammonia-diesel HPDF engine was built after calibrating the spray submodels against experimental ammonia spray data. The ammonia/diesel HPDF engine model was performed with the pilot diesel, and liquid ammonia was injected into the cylinder at −8 and − 5°CA ATDC, respectively. The authors compared the LPDF and HPDF modes, as shown in Figures 7 and 8. In HPDF mode, the ammonia energy share can reach up to 97%. The results revealed that the HPDF mode had equivalent indicated thermal efficiency, cooling, and exhaust loss to the pure diesel mode, but it may significantly reduce greenhouse gas emissions (CO₂ and nitrous oxide (N₂O)) with minor increases in NH₃ emissions.

Figure 7.

Comparison of the energy balances among the pure diesel, HPDF and LPDF modes, reprinted from [32].

Figure 8.

Comparison of the emission characteristics among the pure diesel, HPDF, and LPDF modes, reprinted from [32].

Zhang et al. [33] explored the direct injection of liquid ammonia using experimental measurements on a two-stroke, low-speed CI engine. The authors investigated the combustion and emission characteristics of the diesel/ammonia dual direct injection strategy under different ammonia injection amounts and injection timing of ammonia and diesel. Diesel was injected at −8°CA ATDC, and the liquid ammonia injection timing was adjusted to −16, −8, and 0°CA, corresponding to the three different combustion modes of ammonia, including premixed combustion, premix-diffusion co-combustion, and diffusion combustion. The results demonstrated that injection timing is critical in ammonia ignition to control the combustion phase and duration. The shortest combustion duration and maximum indicated thermal efficiency of 38.8% were found for ammonia injected at −8°CA ATDC, but NO_x levels were high. A certain amount of ammonia diffusion combustion is important for enhancing ammonia combustion. Overall, soot and CO emissions were reduced, presumably because the liquid ammonia direct injection can improve the interaction of spray plumes inside the current combustion chamber, promoting atomization and evaporation of diesel. Injection timing of −8°CA ATDC was the best option for both diesel and liquid ammonia to balance the engine efficiency and emissions. The ammonia energy share in this work was only 50%, with a higher ratio hopefully to be achieved in the future.

2.3 Ammonia-DME dual-fuel operation

The researchers investigated the performance of CI engines using ammonia and DME because of the miscible nature of the two substances [42, 43, 44]. In comparison to ammonia, DME has a greater cetane number, a lower ignition temperature (350°C), a lower latent heat of vaporization (467 kJ/kg), and a higher lower heating value (LHV) (28.43 MJ/kg). It can be generated using renewable energy sources. Ammonia/DME combinations are commercially employed for refrigeration, making their distribution as an alternative fuel more practical [45]. On a single-cylinder, direct-injection diesel engine, Ryu et al. [42] tested three ammonia/DME mixtures (100% DME, 60% DME, and 40% DME). The injection pressure was kept at around 206 bar, and engine combustion and exhaust emissions were evaluated to compare the performance of various ratios of mixture compositions. The findings showed that the engine performance decreased when ammonia was added to the blend fuel mixture. There were noticeable cycle-to-cycle changes when 40% DME + 60% NH₃ was used. It is observed from Figure 9 that the injection timing for successful engine operation needs to be advanced as the ammonia content in the fuel mixture increases. As shown in Figure 10, when ammonia was used in the blended fuel, various exhaust emissions increased. Although NO_x emissions were higher in the case of fuel mixtures containing ammonia compared to pure DME, the emissions remained well within Environmental Protection Agency regulations for small output engines (7.5 g/kWh), and soot emission levels were also quite low (less than 0.002 g/kWh) in all cases. The energy cost of ammonia-DME combinations was equivalent to diesel, and the possibility of producing them using renewable energy sources contributes to a reduction in carbon footprint [44].

Figure 9.

Range of possible injection timing for successful combustion using different fuel mixtures, reprinted from [42].

Figure 10.

Emissions for various fuel mixtures, reprinted from [42].

2.4 Ammonia and other fuels

Gray et al. tested dimethyl hydrazine and amyl nitrate as alternative fuels for diesel in an ammonia dual-fuel CI engine [28]. The results showed that the high cetane number of the test fuel had a significant impact on the engine combustion characteristics. In Ref. [20], soy-based biodiesel was adopted as the ignition source by Reiter and Kong. The experimental results for the diesel-ammonia operation and the biodiesel operation were comparable. Lower NO_x emissions were observed when ammonia content was up to 70%, and using biodiesel also helps to sequester carbon.

3. Advanced combustion technologies in ammonia CI engines

3.1 Ammonia-hydrogen HCCI engines

Low-temperature combustion (LTC) is a series of advanced combustion technologies that promise savings in both fuel usage and pollutant emissions. HCCI is one of the earliest kinds of LTC and arguably the most extensively explored. A homogenous mixture of air and fuel was compressed until it autoignites in HCCI. HCCI has achieved extremely low NO_x and soot emissions while retaining or exceeding traditional diesel combustion efficiency [46]. Only a few studies have been conducted in recent years on the use of ammonia in HCCI engines. Unfortunately, the low power output of HCCI engines, along with ammonia’s low lower heating value (LHV) and the high intake temperature required, reduces the power density of such an application. However, it may be highly appealing for stationary applications such as combined heat and power plants, where great efficiency is achievable.

In Ref. [47], pure ammonia was proven for HCCI engine operation but with a CR of 40:1. Due to the high autoignition resistance of ammonia, hydrogen is required to promote and stabilize HCCI combustion. Pochet et al. developed an HCCI engine operating with ammonia and hydrogen blends under a variable blending ratio [48]. With the addition of hydrogen, the required CR was reduced to 16:1. The results showed that the required intake temperature for pure hydrogen combustion was about 154°C, and there was little impact on autoignition resistance until the ammonia share reached 60%. The engine operated HCCI conditions with an ammonia content of up to 70% by setting the intake pressure to 1.5 bar and the intake temperature to 202°C. The authors were one of the earliest research groups to use exhaust gas recirculation to reduce NO_x emissions from an ammonia-fueled engine [48]. To reduce N₂O generation, measures to compensate for the associated decrease in combustion efficiency and deploying a selective catalytic reduction (SCR) system would be necessary. In the other engine with a CR of 22:1, they were able to burn an NH₃-H₂ blend, with ammonia content varying from 0 to 94% in the fuel blend [49].

3.2 Ammonia cracking and hydrogen fumigation

One advantage of ammonia as an energy carrier is that it may be utilized directly as fuel or cracked and supplied as a hydrogen source. Gill et al. [50] first investigated the effects of providing ammonia, dissociated ammonia (1–2% NH₃, 75% H₂, and 23–24% N₂), and hydrogen to a dual-fuel diesel engine in order to reduce the engine’s carbon-based emissions. Three fuels were supplied directly into the intake manifold, respectively, replacing roughly 3% of the intake air. Diesel was injected directly into the cylinder to ignite the mixture. The results showed that the braking thermal efficiency of all three operations was lower when compared to the pure diesel operation. Among the three distinct ammonia use procedures studied, the application of pure hydrogen was the best for reducing emissions and improving engine performance. However, it is challenging to isolate hydrogen from the dissociated NH₃ mixture on a vehicle. Thus, the better strategy is to partially dissociate the NH₃ to enhance combustion and reduce engine emissions. Using dissociated NH₃ minimizes NH₃ slip and N₂O formation during combustion.

The possibility of ammonia exhaust gas reforming for hydrogen production used shown in transportation was investigated for the first time by Wang et al. [51]. The investigation began with an examination of ammonia autothermal reforming, which combined selective oxidation of ammonia (into nitrogen and water) and ammonia thermal decomposition over a ruthenium catalyst with air as the oxygen supply. Later, the air was replaced with diesel engine exhaust gas to supply the oxygen required for the exothermic reactions that raise the temperature and enhance NH₃ decomposition. The specific experimental setup is shown in Figure 11. The catalytic decomposition of NH₃ requires a temperature higher than 500°C to produce steady NH₃ conversion and substantial H₂ generation. Eq. (3) and Eq. (4) express the selective catalytic oxidation of ammonia and NH₃ decomposition processes, respectively. Eq. (5) shows the desired combination.

Figure 11.

CI engine with a modified ammonia autothermal reforming system, reprinted from [51].

$$4N{H}_3+3O_2=6H_{2}O+2N_2;∆H_r=-1260\mathit{kJ}/\mathit{mol}$$E3

$$2N{H}_3=3H_2+N_2;∆H_r=+46\mathit{kJ}/\mathit{mol}$$E4

$${\mathit{NH}}_3+x{O}_2=2x{H}_{2}O+\left(1.5-2x\right)H_2+0.5N_2;x<0.75$$E5

The hydrogen production efficiency and reforming process efficiency of the NH₃ exhaust gas reforming are shown in Figure 12. Although increasing O₂/NH₃ ratios resulted in higher H₂ generation, the reforming process efficiency declined due to increased NH₃ consumption in exothermic oxidation. The authors added reforming products (H₂ and unconverted NH₃) to the intake port and utilized them as additives to diesel fuel for combustion to explore how different components of reforming products impact engine performance and emissions. Because the NH₃ was not effectively combusted, the addition of the reformate reduces brake thermal efficiency. In terms of engine emissions, replacing diesel with a noncarbon-based reformate reduced carbon emissions but increased NO_x emissions.

Figure 12.

H₂ efficiency and reforming efficiency at 16 g catalyst and a 3 L/min NH₃ flow rate, reprinted from [51].

In the above study, ammonia was used more as a hydrogen carrier to produce hydrogen, and the addition of hydrogen to a diesel-fueled engine can significantly reduce HC, CO, and CO₂ emissions. However, the addition of ammonia may have adverse effects on CI engines. The engine should be improved to take advantage of the potential of ammonia reforming into hydrogen systems.

4. Exhaust emissions in ammonia CI engines

As discussed above, adding ammonia to diesel engines can reduce carbon emissions, but it will also inevitably produce high levels of NO_x and unburned NH₃ emissions. Advanced combustion strategies are beneficial for reducing emissions, but after-treatment measures are still necessary. NO_x, such as NO and nitrogen dioxide (NO₂), contribute to the destruction of the ozone layer and lead to respiratory issues. Nitric acid causes acid rain when NO is combined with atmospheric air and NO₂, which causes smog in and around cities. Due to the fact that fuel and air are premixed in SI engines, the combustion chamber temperature is consistent with flame propagation, resulting in a NO₂/NO_x ratio of less than 2%. However, the combustion in a CI engine is regulated by mixing and has a wide distribution of cold areas and a larger NO₂/NO_x ratio. N₂O is a severe concern since it has a 300-fold greater potential to cause global warming than CO₂ over a 100-year time horizon. The ammonia itself is poisonous, and the quantity of unburned ammonia in the exhaust may cause issues for automobiles.

For the development of efficient abatement technologies, a deeper understanding of nitrogen chemistry during ammonia combustion is required. Miller and Bowman [52] researched the chemistry of nitrogen in combustion, focusing on the production and destruction of NO_x by examining the reaction rates and reaction paths of the primary routes of NH_x radicals. Their work developed the first detailed mechanism for ammonia combustion that was confirmed. After decades of study, researchers now have a thorough understanding of the chemical kinetics of ammonia. They focused not only on the combustion of pure ammonia but also on the co-combustion of ammonia and other fuels. Feng et al. [53] prepared an NH₃-diesel mechanism with complex diesel substitutes (n-cetane, iso-cetane, and 1-methylnaphthalene) and achieved good validation. Figure 13 shows the reaction path of ammonia-diesel at low temperatures. However, there is still insufficient understanding of the cross-reaction between ammonia and diesel. In order to improve the accuracy of the mixing mechanism, it is necessary to conduct experiments and quantum calculations on the cross-reaction.

Figure 13.

Main reaction pathways of NH₃/diesel mixtures, with the initial condition of ϕ = 1.0, pc = 15 bar, and Tc = 690 K, reprinted from [53].

Ammonia burns slowly and has a long quenching distance, making it challenging to completely burn it in the engine. As a result, more unburned ammonia is frequently left in the crevice volume, which increases unburned ammonia emissions. The engine architecture (piston design, crevice, compression ratio, and squish zone for CI engines) plays an important role in the ammonia slit. When gaseous ammonia is used as a fuel, the formation of unburned ammonia is greater as compared to liquid ammonia [54]. Unburned ammonia in the exhaust stream may be beneficial for the operation of the SCR system, which reduces NO_x emissions to nitrogen in the presence of a catalyst, as shown in Eqs. (6) and (7):

$$4\mathit{NO}+4N{H}_3+O_2\to 4N_2+6H_{2}O$$E6

$$6N{O}_2+8{\mathit{NH}}_3\to 7N_2+12H_{2}O$$E7

The Lean NO_x Trap (LNT) may be used to absorb NO and NO₂ regardless of the oxygen exhaust concentration. However, LNT only achieves partial N₂O adsorption-conversion and generates NH₃ as a bi-product during regeneration. Recent studies have provided practical evidence of the beneficial combination of LNT and SCR in series: the fuel penalty can be mitigated when compared to pure LNT, the ideal operating temperature of both systems is more flexible and in the 200–300°C range, and total conversion of NO_x–N₂O–NH₃ is achieved [55]. Three-way catalytic converters can also be used to convert NO_x emissions.

5. Conclusions

Green ammonia is one of the key strategies for diesel engines to attain carbon neutrality. The evolution of ammonia combustion technologies in diesel engines has been reviewed. Not only ammonia is a hydrogen carrier, but it is also readily available, easy to store and transport, and has an established network for transportation and distribution globally by pipeline and bulk carrier. Ammonia as a fuel in CI engines is hampered by its high autoignition temperature and long ignition delay time, resulting in low engine performance. References suggest that the combustion of ammonia in CI engines can be achieved by several strategies: (1) blending with other fuels; (2) higher CR, preheating, or supercharging; (3) high-pressure direct injection of liquid ammonia; (4) ammonia dissociation. It has been successfully demonstrated that highly reactive fuels, including diesel, biodiesel, hydrogen, and DME, may ignite ammonia in CI engines. Ammonia-diesel dual-fuel CI engine operation with ammonia rates up to 95% can be found in the literature. Blending ammonia with traditional fuels reduces the requirement for engine modifications (material compatibility), ensuring cost-effectiveness for the transition to a hydrogen economy.

Due to ammonia’s lower energy density and calorific value, the increased ammonia content in blend fuels reduced carbon emissions but had a negative impact on engine performance. Additionally, ammonia dual-fuel combustion currently produces relatively high levels of unburned ammonia and NO_x emissions because of the fuel-bound nitrogen. Researchers have actively explored solutions to these problems. The use of advanced injection strategies for the secondary fuel can contribute to enhanced performance and overall emissions improvement. Compared to port injection, liquid ammonia direct injection may achieve greater combustion efficiency, and the energy share of ammonia is expected to rise to 97%. Partial dissociation of ammonia contributes to improved combustion performance. Ammonia-hydrogen combustion in HCCI engines is also a viable technical route. The use of various advanced strategies may reduce NO_x and unburned NH₃ emissions, but posttreatment measures such as SCR systems are still necessary. It will be more cost-effective and convenient to reduce emissions by using unburned ammonia emissions in the exhaust stream rather than a separate urea/ammonia injection. Ammonia as a compression ignition fuel is currently only considered a viable option for maritime, power-generating, and heavy-duty applications without serious space restrictions.

There is currently limited literature on ammonia combustion in CI engines, and more research is needed to grow the use of ammonia as a hydrogen energy carrier from an incipient stage to one that is well-established and commercially viable. Although numerical simulations are already a well-respected method for investigating ammonia combustion in engines, more research is still needed to provide reliable data and enhance the simulation tools’ ability to forecast how future ammonia engines will be designed and optimized. The potential of ammonia in advanced CI engines should also be considered in the future, including homogeneous charge compression ignition, premixed charge compression ignition, and reactivity-controlled compression ignition.