Review of combustion technologies for ammonia-diesel compression ignition engines: Approaches to achieving high ammonia substitution

Abstract

Ammonia, as a zero-carbon fuel, has great potential for meeting decarbonization targets in the internal combustion engine sector. This paper summarizes recent studies in which ammonia is used as a fuel for compression-ignition engines. Due to its low combustion reactivity, ammonia must be used in conjunction with a high reactivity fuel, such as diesel, to ensure stable engine operation. Currently, two main approaches are used to supply ammonia to the engine combustion chamber: ammonia port injection and in-cylinder direct injection. In the two routes, ammonia-diesel engines commonly face challenges such as low ammonia energy rate (AER), limited thermal efficiency, and high emissions of nitrogen-containing pollutants, especially under high ammonia substitution conditions. To address these challenges, this study reviews combustion technologies capable of achieving relatively high AER, such as premixed charge compression ignition (PCCI) and reaction-controlled compression ignition (RCCI), and analyzes their impact on combustion and emissions characteristics. This paper also examines combustion technologies under ultra-high AER conditions and finds that technologies such as diesel pilot injection and ammonia-diesel stratified injection can support stable engine operation. This review provides insights into current progress, remaining challenges, and future directions in ammonia-diesel engine combustion technologies.

Graphical abstract

Keywords

dual fuel engine / ammonia energy rate (AER) / compression ignition / combustion technology / emissions control

1 Introduction

Global warming and energy shortage necessitate a transition of the global energy structure toward low-carbon, efficient, and clean solutions [1]. Over the past three years, carbon emissions from the transportation sector accounted for 18.9%, 19.6%, and 20.1% of the total global carbon emissions, respectively [2]. Carbon dioxide is the most impactful greenhouse gas driving global temperature increases, contributing to approximately 30% of climate warming [3]. According to the Paris Agreement (COP21), reducing carbon emissions in the transportation sector is a crucial component of the global effort to achieve carbon reduction targets [4–10]. Within this sector, internal combustion engines play a crucial role, particularly in long-distance heavy-duty trucks, maritime transport, and other applications [11]. The use of zero-carbon and carbon-neutral fuels in internal combustion engines, along with the development of energy-saving and emission-reduction technologies, holds transformative strategic significance for achieving carbon neutrality in both the transportation and energy sectors [12–15].

Ammonia, as a fuel for powertrain systems, possesses several advantageous properties, including a hydrogen content of 17.7% [16–18], low production cost [19], and well-established production technology [20]. As a zero-carbon fuel, ammonia can make a unique contribution to the energy transition by significantly displacing conventional fossil fuels. Figure 1 presents an overview of the production, storage, transportation, and application of ammonia. Currently, ammonia is produced via the Haber-Bosch method, solid-state ammonia synthesis, and photocatalytic ammonia production, methods that can result in zero carbon emissions throughout the entire production and consumption cycle [21,22]. Ammonia can be completely liquefied at 298.15 K and 0.9 MPa and is primarily stored and transported in its liquid state. It can be utilized in a variety of applications, including maritime transport, heavy-duty trucks, power generation, and fuel cells. Consequently, ammonia fuel holds strong potential as a viable solution for the green transformation of future energy systems [23].

In heavy-duty engines, diesel remains the predominant fuel for compression ignition engines. Using zero-carbon fuels within the existing framework of diesel engines can help reduce the costs associated with energy system transformation [24,25]. Due to ammonia’s excellent anti-knock properties [26,27], higher compression ratios can be employed, thereby improving engine thermal efficiency [28]. Compression ignition engines offer superior torque characteristics and a wider power range, making them more versatile compared to spark ignition engines. Therefore, ammonia fueled engines, in which ammonia is ignited using highly reactive fuels, hold significant application potential [29]. Among these, dual-fuel compression ignition engines that use diesel to ignite ammonia have become a key area of research. Table 1 summarizes representative studies on ammonia-diesel engines with high ammonia energy ratios (AERs), including engine specifications, test conditions, and main findings. Further details of these studies will be introduced later.

Although stable combustion in ammonia-diesel engines can be achieved through various strategies, research focused on combustion technologies for ammonia-diesel engines operating at ultra-high AER remains relatively limited. Since an increased ammonia energy ratio leads to reduced carbon emissions, identifying methods to increase the AER is a key challenge in the development of ammonia-diesel engines. This paper reviews the technical pathways of ammonia-diesel dual-fuel compression ignition engines, with a particular focus on combustion technologies at high AER. The development goals for ammonia-diesel engines include achieving high AER, high thermal efficiency, and low emissions. Reaching these goals requires adjustments to both the reactivity and spatial distribution of the air/fuel mixture. Combustion modulation strategies that could be used to achieve these goals are summarized in Fig. 2. Methods to improve mixture reactivity include direct injection of a more reactive diesel-type fuel, the use of turbulent jet combustion prechambers, and the increase of the intake air temperature. Techniques to locally stratify the mixture reactivity include optimization of injection timing and strategies (e.g., multiple injections), variable valve timing to control local dilution via internal exhaust gas recirculation, and external exhaust gas recirculation.

2 Ammonia fuel properties

The physicochemical properties of a fuel significantly influence its combustion behavior. A comparative analysis of physico-chemical properties of ammonia with conventional fuels, as presented in Table 2, highlights its distinctive features. Although ammonia has a relatively low energy density (18.6 MJ/kg) [18], its considerably lower stoichiometric air/fuel ratio compared to typical hydrocarbon fuels allows ammonia-air mixtures to possess higher energy content per unit mass of air. Understanding these fundamental combustion characteristics of ammonia is essential for the effective design of ammonia-based combustion systems.

Previous research has investigated the combustion properties of ammonia [41,42], identifying several inherent limitations, including a high auto-ignition temperature [43], low laminar flame speed [44–46], and poor combustion stability [47–49]. These adverse properties result in suboptimal combustion performance and impose significant constraints on the practical application of ammonia as a primary fuel. For instance, achieving ignition with pure ammonia in compression-ignition engines necessitates compression ratios over 35 [50]. Such elevated compression ratios substantially increase mechanical and thermal stresses during engine operation, thereby raising the probability of engine failure [51]. Consequently, employing highly reactive fuels as ignition agents has become a requisite to enable the application of ammonia in compression ignition engines [52].

To facilitate the deployment of ammonia as a viable engine fuel, it is imperative to investigate the fundamental kinetic mechanisms governing its combustion [53]. The current understanding of ammonia combustion mechanisms primarily relies on the detailed reaction pathways proposed by Miller et al. [54]. Among the feasible strategies for ammonia utilization, diesel-assisted ignition has gained attention, leading to the development of dual fuel combustion mechanisms. However, earlier combustion models often overlooked C‒N interactions, limiting their ability to accurately simulate ammonia-diesel combustion processes [55]. To address this limitation, Zhang et al. [56,57] incorporated C–N interaction pathways into their models, thereby enhancing the predictive accuracy of ignition delay times, particularly under fuel-lean conditions. Table 3 shows the reactions relevant to NH₃/diesel combustion [56]. Building on this work, Sun et al. [58] further refined the rate constants for C–N interactions, enabling more reliable predictions of ignition delay times and laminar flame speeds across a broad pressure range. Lai et al. [59] subsequently simplified the ammonia-diesel reaction mechanism, effectively modeling the high-temperature flame propagation. Figure 3 depicts the chemiluminescence images of 60% AER and 80% AER, alongside the corresponding predicted temperature distributions in the cylinder at similar crank angles. In the visualized chemiluminescence image, a fuel-rich zone is formed in the diesel injection region, and fuel combustion produces a high-temperature yellow-white flame. The simulated high-temperature zone coincides with the high-temperature flame distribution, enabling the prediction of in-cylinder combustion and temperature changes in the RCCI engine model.

The combustion of ammonia produces nitrogen-containing pollutants, including nitrogen oxides (NO_x) such as NO, NO₂, and N₂O, as well as unburned ammonia [60,61]. The formation of thermal NO_x is primarily governed by the Zeldovich mechanism [62–64], while intermediates such as HNO and NNH intermediates play crucial roles in the generation of fuel-derived NO_x [65]. In ammonia-diesel engines, ammonia combustion primarily depends on diesel fuel ignition. However, complete combustion of ammonia is not achieved in regions with low diesel concentration [66]. Since N₂O emission mainly results from the low-temperature oxidation and incomplete combustion of ammonia, high N₂O emissions are often correlated with high unburned ammonia emissions [67].

3 Technologies for ammonia compression-ignition engines

Ammonia can be supplied to compression-ignition engines through two main methods: ammonia port injection and ammonia direct in-cylinder injection. Figure 4 illustrates schematic diagrams of an ammonia port injection configuration and a dual-fuel direct in-cylinder injection configuration. The primary distinction between the two methods lies in how ammonia is introduced into the combustion chamber. In ammonia port injection, ammonia is premixed with air before entering the combustion chamber, and the spatial distribution of the diesel injection determines the ignition location. In contrast, ammonia direct in-cylinder injection requires consideration of the interaction between the ammonia spray and the diesel spray. The high latent heat of vaporization of ammonia may further increase the difficulty of achieving reliable ignition in this configuration.

3.1 Compression-ignition engines with ammonia port injection

Ammonia port injection delivers ammonia into the intake manifold, where it mixes with air to form a mixture that enters the cylinder during the intake stroke. As the piston approaches top dead center, diesel is injected into the cylinder, initiating ignition and subsequent combustion. The combustion process typically involves multiple stages, including diffusion combustion of the diesel and flame propagation of the ammonia-air mixture [68]. Ammonia port injection is favored due to its simplicity in engine system modification.

However, increasing the AER by port injection, without employing additional combustion enhancement techniques, can adversely affect engine performance. Reiter and Kong [31] experimentally observed that increasing AER through ammonia port injection led to a significant drop in fuel conversion efficiency. At 95% AER, the efficiency was reduced by two-thirds, compared to that observed at 60% AER. Yousefi et al. [30,32] investigated the combustion and emissions of an ammonia-diesel engine with AER ranging from 0% to 40%. With ammonia port injection, increasing AER results in a higher concentration of ammonia in the premixed intake air. However, the ammonia mixture remains difficult to ignite away from the diesel spray, resulting in a drop in engine thermal efficiency from 38.1% to 37.2%. However, by adopting a diesel split injection strategy, the thermal efficiency was increased to 39.7% at 40% AER.

While ammonia use effectively reduces carbon emissions, it presents challenges related to NO_x and unburned ammonia emissions. Therefore, optimizing in-cylinder combustion conditions is essential for mitigating these emissions. Kurien and Mounaïm-Rousselle [69] demonstrated that unburned ammonia emissions in ammonia-diesel engines are minimized under stoichiometric conditions, where optimal combustion temperature and flame propagation promote complete ammonia oxidation. In contrast, under lean or rich conditions, deviations from the stoichiometric fuel-air ratio reduce combustion efficiency and increase unburned ammonia emissions. Moreover, Ichikawa et al. [70] found that higher-temperature combustion helps reduce N₂O emissions. Further studies [71] revealed that under high-load conditions, elevated combustion temperatures lead to a more complete combustion process, reducing unburned ammonia emissions by 11% compared to the low-load conditions.

3.2 Compression-ignition engine with ammonia direct injection

The application of ammonia direct injection primarily follows two approaches. One involves low-pressure gaseous ammonia injection into the cylinder in the early intake stroke using a specific ammonia injector. The other method involves the direct injection of liquid ammonia. Research on gaseous ammonia direct injection remains limited, with most studies focusing on spark-ignition ammonia engines [72,73]. In contrast, direct injection of liquid ammonia facilitates precise adjustment of the mixing ratio and optimization of the combustion process, making it a promising approach for compression-ignition engines.

Direct injection of liquid ammonia may induce a flash boiling phenomenon at the nozzle outlet, potentially resulting in non-uniform spray characteristics. It is, therefore, important to study the liquid ammonia spray behavior to understand the performance of ammonia-diesel direct in-cylinder injection engine. Zhang et al. [74] investigated the liquid ammonia/air mixing characteristics under direct in-cylinder injection conditions and found that the Lagrangian model can accurately capture spray behavior of liquid ammonia under non-flashing conditions, but its predictive performance is less reliable when flash boiling occurs. To further examine spray evolution near the nozzle, Zhang et al. [75] numerically studied the spray penetration behaviors of liquid ammonia injection involving phase change. As shown in Fig. 5(a), liquid ammonia penetration increased with reduced ambient pressure, while ammonia vapor penetration increased with decreased ambient density. Additionally, as shown in Fig. 5(b), the lower boiling point of liquid ammonia results in a higher evaporation rate compared to diesel, leading to a shorter tip penetration distance but a wider spray angle [33].

The direct liquid ammonia injection strategy plays an important role in mixing ammonia with diesel and air in the combustion chamber. Nyongesa et al. [76] found that by installing an ammonia injector in the engine cylinder head and adjusting the injection direction, timing, and pressure, AER as high as 80% could be achieved, resulting in an 80% reduction in CO₂ emissions. Li et al. [77] found that high-pressure liquid ammonia injection maintained indicated thermal efficiency similar to that of diesel combustion, while also reducing unburned ammonia and NO_x emissions due to thermal denitrification occurring in the combustion chamber. Zhang et al. [33] used the diesel jet-controlled compression ignition (JCCI) technique to ignite liquid ammonia and found that the combustion phase of ammonia is highly sensitive to diesel injection timing. Bjørgen et al. [34] investigated the effect of liquid ammonia injection timing on engine performance by fixing the diesel injection moment at −15 °CA after top dead center (ATDC), and found that ammonia injection between −80 and −60 °CA ATDC resulted in a substantial increase in unburned ammonia emissions, whereas delaying ammonia injection to −30 °CA ATDC promotes premixed combustion. The optimal injection strategy involved overlapping ammonia and diesel injection timings, resulting in combustion efficiencies up to 86%.

4 Compression-ignition engine combustion technology with high AER

Degradation of combustion efficiency and increased pollutant emissions are particularly pronounced under high AER conditions [78]. Sun et al. [79] using optical engine experiments, observed that the peak flame area ratio (FR) and flame natural luminosity (FNL) decreased by 60% and 92%, respectively, when AER reached 80%. This indicates that high AER significantly attenuated flame development in the combustion chamber. To achieve efficient and clean combustion at high AERs, improvements have been made to ammonia-diesel engines, such as optimization of the combustion chamber geometry, adoption of advanced injection techniques, and enhancement of ignition fuel activity, which will be discussed in this section.

Fuel spatial distribution in the combustion chamber strongly affects combustion and flame propagation in ammonia-diesel engines. Optimizing the combustion chamber geometry and injector placement can change fuel spatial distribution to improve combustion performance [80]. Sehili et al. [81] identified the optimal piston bowl geometry for 85% AER operation. Figure 6 shows the effect of the piston geometry on the combustion process, with τ_comb denoting the percentage of fuel burned. Their study showed that the piston-cylinder boundary significantly affects flame propagation. By optimizing the geometrical parameters shown in Fig. 6(a), engine-out unburnt ammonia emissions were reduced by 43%, while thermal efficiency improved by 31%. Cui et al. [82] investigated the effects of injector positioning on the engine performance in a liquid ammonia-diesel direct in-cylinder injection engine, and found that the relative angle between diesel and ammonia injectors controls ammonia ignition timing. An angle of 75° between injectors resulted in a peak thermal efficiency of 43.6% at 80% AER. Thermal efficiency tended to decrease when ammonia and diesel nozzles were positioned too close to each other.

Flexible fuel injection strategies can optimize the fuel mixing and distribution in the combustion chamber by adjusting injection timing, injection pressure, and injection segments [83,84,35]. These flexible injection technologies enable advanced combustion strategies such as premixed charge compression ignition (PCCI) and reaction-controlled compression ignition (RCCI). In the PCCI strategy, diesel fuel is generally injected in the early compression stroke, allowing sufficient time for mixing ammonia with diesel to form a more homogeneous mixture. PCCI is especially suitable for low to intermediate load conditions, though ignition control remains a challenge. In the RCCI strategy, diesel fuel is injected in the late compression stroke, creating an in-cylinder reactivity stratification, characterized by a high-reactivity diesel-rich region and a low-reactivity premixed ammonia/air region. Diesel injection control in RCCI allows flexible adjustment of the combustion phase and burn rate, enabling engine operation over a wide load range.

The PCCI technique mixes ammonia and air well before the compression stroke to form a homogeneous premixed gas. During compression ignition, this premixed ammonia-air mixture is ignited by diesel fuel, resulting in premixed combustion [85]. Pei et al. [36] found that advancing the diesel injection timing in low load PCCI mode extended the mixing time between diesel and ammonia appropriately, resulting in a more homogeneous fuel mixture. Indicated thermal efficiency reached up to 49.5% when diesel injection occurred at −40 °CA ATDC. Wu et al. [37] found that optimizing PCCI combustion in ammonia-diesel engines lies in tuning both the reactivity and spatial distribution of the fuel blend. Increasing the ammonia energy fraction decreases overall fuel mixture reactivity, while proper diesel injection timing improves fuel mixture uniformity, and higher diesel injection pressure enhances mixing before ignition. The reasonable adjustment of these factors can effectively improve the engine thermal efficiency. Zi et al. [86] further investigated PCCI combustion using a diesel split injection strategy. Effects of the injection strategy on in-cylinder OH radical distribution and heat release are illustrated in Fig. 7. Diesel split injection significantly improved mixture reactivity distribution at CA10, as shown in Fig. 7(b), leading to a notably expanded OH radicals distribution. Compared with single-injection diesel, split injection advanced combustion timing and accelerated the burn rate, promoting a fast pressure rise. This improved spatial distribution of diesel fuel resulted in a more homogeneous OH radical formation, ensuring rapid low- and high-temperature reactions and promoting complete ammonia combustion. As a result, an indicated thermal efficiency of 49.3% was achieved in PCCI combustion mode.

Reaction-controlled compression ignition (RCCI) technology improves combustion characteristics by injecting two fuels with different reactivities into the combustion chamber. The mixing ratio and injection timing of these fuels are adjusted to produce different mixing reactivity stratification within the cylinder. In ammonia-diesel RCCI engines, ammonia is supplied into the engine early in the intake stroke to premix thoroughly with the air, reducing the overall mixture reactivity and resulting in a milder combustion process. Diesel fuel is then injected late in the compression stroke as an ignition source to ensure stable combustion [87,88]. Xu et al. [89] investigated the combustion behaviors of an ammonia-diesel dual-fuel engine and found that RCCI combustion occurs in two stages: the first stage involves a diffusion flame driven by diesel combustion, and the second stage is a premixed combustion of the ammonia/air mixture. Niki [90] examined the effect of diesel injection timing on engine emissions, reporting that early diesel pre-injection effectively reduced NH₃ and N₂O emissions but led to an increase in NO_x, HC, and CO emissions in ammonia-diesel RCCI engines. Fakhari et al. [91] numerically analyzed the effect of intake temperature on combustion and emissions of ammonia-diesel RCCI engines. Figure 8(a) shows that at 80% AER, increasing intake temperature raised both peak cylinder pressure and peak heat release rate, which also occurred earlier. Figure 8(b) shows that higher initial charge temperatures facilitate earlier formation of OH radicals (indicated by increased OH concentration), indicating an earlier combustion start.

Premixing highly reactive gaseous fuels, such as hydrogen or ozone, with ammonia improves the reactivity of the mixture, reduces the reliance on pilot diesel, and facilitates stable combustion at high AER. Sehili et al. [81] showed that adding a small amount of hydrogen improved combustion reactivity under high load and high AER conditions, leading to improved thermal efficiency and stable operation of the ammonia-diesel engine. Xu et al. [92] supplied premixed ammonia and hydrogen into the combustion chamber and found that hydrogen accelerated the ammonia combustion rate in the RCCI engine. However, when the energy share of hydrogen exceeded 30%, the ammonia-hydrogen mixture spontaneously ignited during the compression stroke before diesel injection, resulting in a 60% increase in NO emissions compared to the hydrogen-free case. Lang et al. [93] studied the effect of ozone addition on ammonia-diesel RCCI engine performance at low load. Figure 9 shows flame development images at different ozone concentrations. Ozone effectively replenished the OH radical pool during combustion, resulting in faster flame propagation, increased flame area, and greater flame brightness. Under the test conditions, adding 1200×10^–6 ozone increased IMEP by 12%, while 1600×10^–6 ozone reduced ignition delay by over 50%.

5 Compression-ignition engine combustion with ultra-high AER

The dependence of ammonia-diesel engines on fossil fuels can be reduced by minimizing the amount of diesel used, making it essential to explore methods for ensuring stable combustion at ultra-high AER. In this study, ultra-high AER is defined as an ammonia energy fraction exceeding 90% of the total fuel energy. The key to stable operation of ammonia-diesel engines at ultra-high AER is the use of a minimal amount of diesel fuel to ignite the ammonia [94]. Zheng et al. [95] used an optical rapid compression machine to study the premixed ammonia combustion process initiated by diesel ignition. Figure 10 shows the ignition and combustion behavior of ammonia-diesel engine at different AERs. Their results show that lower diesel injection pressures or larger diesel nozzle diameters facilitate rapid mixing of diesel with ammonia near the nozzle, significantly shortening ignition delay at 95% AER. Liu et al. [96] advanced diesel fuel injection timing to create a high-temperature reactive environment in the combustion chamber. Numerical simulation showed that this reactive environment supports stable ignition at AERs exceeding 90%.

Other notable approaches to achieving ultra-high AER in ammonia-diesel engines include turbulent jet combustion chamber (TJCC) technology and ammonia-diesel stratified injection. The TJCC improves fuel mixing and ignition by introducing strong turbulence effects. In this system, diesel fuel is injected into a pre-chamber and then auto-ignited. The resulting high-temperature, high-pressure combustion products rapidly enter the main combustion chamber through a turbulent jet orifice. This turbulent jet generates intense turbulent mixing in the main combustion chamber, promoting the ignition and combustion of ammonia [97]. In a numerical study, Yang et al. [98] successfully achieved an AER of 90% using an ammonia-diesel engine equipped with a turbulent jet combustion chamber.

Compared to TJCC technology, diesel pilot injection technology does not require modifications to the combustion chamber structure, making it possible to achieve stable engine combustion at ultra-high AER by optimizing the diesel injection strategy. In ammonia-diesel engines, diesel pilot injection typically involves injecting a small amount of diesel fuel (5%–20% of total energy) during the late compression stroke. The auto-ignition of diesel forms a locally high-temperature flame core, which triggers the diffusion combustion of the ammonia/air mixture [99]. Additionally, diesel split injection technology can be used to further optimize combustion and emissions characteristics of ammonia-diesel engines [100]. The following examples illustrate the application of diesel pilot technology at ultra-high AER.

Scharl and Sattelmayer [101] investigated the ignition and combustion characteristics of diesel pilot injection combined with liquid ammonia direct injection in a rapid compression machine. Their findings showed that complete combustion occurs if the liquid ammonia spray contacts the diesel spray after diesel ignition. Conversely, if the liquid ammonia spray contacts the diesel spray before diesel ignition, the ignition process is inhibited, even under full load conditions. Rousselle et al. [38] compared single and double injection strategies in ammonia-diesel engines operating at moderate loads with diesel pilot injection. Both strategies, with diesel fuel contributing less than 10% of the total energy, achieved complete ammonia combustion. The double injection strategy increased local fuel mixture fraction and reactivity, reducing both NO_x and unburned NH₃ emissions.

Qiu et al. [39] conducted an optimization study on the diesel pilot injection strategies for ammonia-diesel engines. As shown in Fig. 11, increasing the AER from 30% to 90% caused the maximum cylinder pressure to decrease from 1.62 to 1.49 MPa. The initial small peak corresponds to pre-injection diesel combustion, while the second peak represents heat release from ammonia-diesel co-combustion. NO_x emissions showed a non-monotonic trend, with the lowest NO_x emissions at 60% AER, suggesting a shift in NO_x formation mechanisms from thermal-dominance to fuel-dominance. By adjusting the pre-injection diesel quantity, main injection timing, and the interval between two injection segments, stable engine operation at an ultra-high AER of 95% was achieved, along with a thermal efficiency of 40.2%. It was also observed that higher engine loads at elevated AER improved thermal efficiency.

Ammonia-diesel engines usually require two independent fuel injection systems for ammonia and diesel, and the in-cylinder spatial distribution of these two fuels is strongly influenced by the relative positions of their nozzles. Improper nozzle positioning can lead to uneven fuel distribution, which negatively impacts combustion performance under high AER conditions. To address this issue, some researchers have explored the use of coaxial injectors to simultaneously inject ammonia and diesel, enabling stratified fuel distribution by adjusting injection timing and strategies for both ammonia and diesel [102]. Takasaki et al. [103] experimentally studied coaxial injection technology using incompatible water and diesel fuel as a proxy, laying a foundation for the application of coaxial injection with ammonia and diesel, which are also immiscible. Further, Wirbeleit et al. [104] and Bedford et al. [105] experimentally optimized stratified fuel injection using coaxial injection technology, demonstrating reductions in NO_x and particulate emissions from diesel engines.

Liu et al. [40,106] conducted simulation studies of ammonia-diesel coaxial injection and stratified injection strategies in low-speed marine engines. Figure 12(a) shows the structure of the ammonia-diesel coaxial injector, and Fig. 12(b) illustrates the stratified injection strategy. In this approach, a single injector delivers both ammonia and diesel at different injection timings, as schematized in Fig. 12(b). Their results indicate that the initially injected diesel creates a high-temperature environment that facilitates ignition, while subsequent multiple diesel injections promote efficient ammonia combustion. Coaxial injection enhances the interaction between ammonia spray and diesel flame zones, ensuring stable ammonia combustion even when diesel ignition energy is as low as 1%. Collectively, these studies show that appropriate injection strategies are critical to improve ammonia-diesel engine performance, maximizing efficiency while minimizing diesel consumption.

6 Summary and outlook

6.1 Summary

The application of ammonia as the fuel for combustion engines presents a promising pathway to decarbonize the road and marine transportation sectors. However, due to the ignition reluctance and slow flame propagation, fuels with higher ignition reactivity, such as hydrogen or diesel-like fuels, are required to trigger ammonia combustion. This paper reviews recent advances in combustion control technologies for ammonia-fueled compression ignition engines, where diesel serves as a combustion promoter.

Figure 13 summarizes the different engine strategies according to ammonia substitution rate levels, highlighting key challenges and the solutions, with an emphasis on combustion technologies for high ammonia substitution rates. Combustion strategies for ammonia-diesel engines differ based on the fuel supply methods. Engines using ​ammonia port injection leverage the ​premixed ammonia-air mixture and rely on optimized diesel injection strategies to control combustion, typically ​without significant engine modifications. In contrast, ​liquid ammonia direct injection necessitates precise coordination​ of the ​temporal and spatial distribution​ of both ammonia and diesel fuels, as well as managing their physical and chemical interactions. This approach demonstrates potential for enhanced thermal efficiency​ and higher ​ammonia substitution rates through refined fuel interplay management.

In ammonia-diesel compression ignition engines, ammonia can be supplied either through intake port injection or through in-cylinder direct injection. Based on the ammonia substitution rate for diesel fuel, these engines are classified as ammonia-diesel dual fuel engines, where the energy shares of diesel and ammonia are comparable, or diesel pilot ignition ammonia engines, where ammonia’s energy share far exceeds that of diesel fuel.

Ammonia port injection technology involves fewer modifications to the engine cylinder head, making it less complex to implement. However, early studies suggested that achieving high ammonia substitution rate via port injection is difficult, and as such studies predominantly focused on dual fuel combustion strategies with ammonia substitution rates usually below 80%. In contrast, liquid ammonia direct injection allows for more flexibility in the spatial distribution adjustment of the diesel-ammonia-air mixture in the combustion chamber. Consequently, research on this technology emphasizes the interaction between diesel and ammonia sprays and their effect on combustion and emissions. A major advantage of liquid ammonia direct injection is its capability to achieve higher ammonia substitution rates and improved thermal efficiency, though challenges remain in redesigning the engine cylinder head and ensuring the durability of the direct injection system.

Since higher ammonia substitution rate correlate with lower CO₂ emissions, recent studies have focused on combustion and emissions characteristics under high or ultra-high ammonia substitution conditions, in which ammonia combustion is triggered by diesel pilot ignition. Achieving higher ammonia substitution requires careful control of the in-cylinder fuel-air mixing and spatial reactivity distribution, which subsequently affect flame development. PCCI and RCCI which promote mixture homogeneity and mixture stratification respectively have been studied to optimize performance at the upper limit of ammonia substitution rate. Existing studies indicate that the diesel pilot injection strategy critically impacts mixture ignition and combustion under ultra-high ammonia substitution conditions. Compared to single-stage injection, split diesel injection shows more robust ignition ability. By optimizing the injection mass distribution and timing between different injection stages, a peak ammonia substitution rate of 95% has been experimentally achieved, which is the highest ammonia substitution rate reported up to date, while maintaining engine thermal efficiency above 40% [39].

6.2 Outlook

Ammonia-diesel compression-ignition combustion is an important technology pathway for utilizing ammonia as a fuel in internal combustion engines. Due to its relatively low requirements for engine structure modifications and minimal changes to existing infrastructure, it is considered an economically viable solution for carbon emissions reduction in heavy-duty road and maritime transportation. However, several challenges remain for the wide application of ammonia-diesel compression ignition engines in the transportation sector, such as maximizing ammonia substitution rates to achieve the greatest carbon emissions reductions, optimizing engine thermal efficiency under ultra-high ammonia substitution conditions, and simultaneously controlling nitrogen-contained emissions like NO_x, N₂O, and unburned NH₃. Continued advancements in combustion control and aftertreatment technologies are essential to address these challenges.

6.2.1 Ammonia-diesel engine combustion technology development

For ammonia-diesel compression ignition engines, achieving a high ammonia substitution rate while maintaining high thermal efficiency is a complementary challenge. Enhancing the use of ammonia as a fuel substitute must be carefully balanced with improving thermal efficiency to ensure overall engine performance. Under elevated ammonia substitution conditions, it is imperative to direct attention toward stable ignition and rapid combustion of ammonia. Both the chemical and physical properties of the pilot ignition fuel have a significant impact on ammonia ignition. For example, differences in the ignition behavior have been observed between diesel and biodiesel fuels with varying cetane numbers [32,107,108]. Therefore, future research should aim to clarify the relationship between diesel properties and ammonia ignition under different ammonia premixed conditions. Based on this understanding, an evaluation method for pilot fuel ignition tendency could be developed, enabling the formulation of diesel fuels specifically tailored for ammonia-diesel combustion engines. Moreover, by integrating pilot-ignition fuel properties with combustion system design and fuel combustion process modulation, advanced engine combustion technology can be developed to address the current limitations on ammonia substitution rates and thermal efficiency in ammonia-diesel engines.

In addition to balancing ammonia substitution rates and thermal efficiency, combustion technologies for ammonia-diesel engines must also control the combustion chemistry to mitigate nitrogenous pollutants. However, the formation mechanisms of nitrogen-containing pollutants at high AERs require further investigation [109]. For example, debates persist regarding the reaction rates of key intermediates such as NNH and HNO in NO_x formation pathways [110]. Additionally, distinguishing the relative contributions of thermal NO_x and fuel NO_x to overall emissions remains challenging [111], while current kinetic models still exhibit limitations in accurately predicting both low-temperature oxidation processes and N₂O generation [112].

6.2.2 Ammonia-diesel engine aftertreatment technology development

Nitrogen-contained emissions such as NO_x and unburned NH₃ represent significant challenges for ammonia fueled engines [113]. Due to the nitrogen inherent in ammonia fuel, elevated levels of these nitrogenous emissions are commonly associated with ammonia combustion. Controlling these emissions cannot rely solely on in-cylinder combustion modulation, making efficient aftertreatment technologies essential for effective emissions management. Selective catalytical reduction (SCR) is an effective method to reduce NO_x emissions from ammonia-fueled engines, with NH₃ emissions in the exhaust gases serving as the reductant for SCR system [114]. However, depending on the engine operational conditions, either NO_x or NH₃ emissions may become excessive, preventing the simultaneous effective reduction of both emissions. When NO_x emissions exceed the SCR system’s processing capacity, supplemental NH₃ supply is required, necessitating real-time aftertreatment control strategies based on ammonia injection as a reducing agent. Conversely, in cases of excessive NH₃ emissions, ammonia slip catalyst (ASC) technology is employed as a supplementary measure to oxidize unburned ammonia into nitrogen gas and water vapor.

N₂O is another key nitrogen-containing emission from ammonia-fueled engines that requires effective mitigation due to its greenhouse potential—approximately 300 times that of CO₂ [69,62]. N₂O is generated both from intermediate products formed during the low-temperature oxidation of ammonia [115] and from incomplete oxidation of unburned NH₃ in aftertreatment systems (such as ASC), as well as from side reactions occurring in SCR systems during NO_x treatment [116]. Given the multiple pathways of N₂O formation, a coordinated control strategy integrating in-cylinder combustion control and aftertreatment catalytic control is necessary. In-cylinder combustion control should focus on minimizing local low-temperature zones prone to N₂O generation by carefully regulating the spatiotemporal distribution of ammonia-diesel mixtures [116]. N₂O aftertreatment catalytic technologies mainly include direct catalytic and selective catalytic reduction (SCR) methods. Direct catalytic approaches employ noble metal or molecular sieve catalysts but suffers from issues such as narrow catalytic temperature windows (500–600 °C), high cost, and thermal sintering risks [117]. SCR technology must simultaneously achieve N₂O elimination and suppression of N₂O formation during catalytic reactions [118]. Key challenges involve overcoming interference from O₂, H₂O, and NO during N₂O removal, precisely regulating catalyst redox properties to prevent excessive NH₃ oxidation when suppressing N₂O formation.

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