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Review

Hybrid Fuels for CI Engines with Biofuel Hydrogen Ammonia and Synthetic Fuel Blends

by
Ramozon Khujamberdiev
and
Haeng Muk Cho
*
Department of Mechanical Engineering, Kongju National University, Cheonan 31080, Republic of Korea
*
Author to whom correspondence should be addressed.
Energies 2025, 18(11), 2758; https://doi.org/10.3390/en18112758
Submission received: 16 April 2025 / Revised: 23 May 2025 / Accepted: 24 May 2025 / Published: 26 May 2025
(This article belongs to the Special Issue Sustainable Energy Development in Liquid Waste and Biomass: 2nd Edition)
Versions Notes

Abstract

:
The transition to sustainable energy systems necessitates the development of cleaner fuel alternatives for compression ignition (CI) engines, which continue to play a vital role in transportation and power generation. This study explores the potential of hybrid fuel blends comprising biofuels, hydrogen, ammonia, and synthetic fuels to enhance engine performance while minimizing environmental impact. By reviewing recent advancements, the paper analyzes the combustion characteristics, emissions behavior, and feasibility of various fuel combinations. Biofuel–hydrogen blends improve flame speed and reduce carbon emissions, while ammonia offers zero-carbon combustion when paired with more reactive fuels, like biodiesel or hydrogen. Synthetic fuels, particularly those derived from renewable sources, provide high-quality combustion with low particulate emissions. Hybridization strategies leverage the strengths of each component fuel, resulting in synergistic effects that enhance thermal efficiency, reduce greenhouse gas emissions, and support the continued use of CI engines in a carbon-constrained future. The findings indicate that with proper optimization of fuel formulations and engine technologies, hybrid fuels can play a key role in achieving sustainability goals and reducing fossil fuel dependency.

1. Introduction

Compression ignition (CI) engines play a crucial role in transportation and power generation, contributing significantly to global energy consumption [1,2,3,4]. However, their reliance on fossil fuels poses challenges, including greenhouse gas emissions and resource depletion. Recent advancements have explored alternative fuels to improve sustainability and reduce emissions [5,6]. Hybrid fuel combinations, integrating biofuels, hydrogen, ammonia, and synthetic fuels, have emerged as potential solutions to enhance engine efficiency and mitigate environmental impact [7].
The growing concern over fossil fuel dependency has driven research toward alternative fuels that offer lower emissions and better sustainability. Biofuels, derived from renewable sources, have shown promise in reducing carbon footprints, while hydrogen and ammonia provide carbon-free combustion options. Synthetic fuels, engineered to mimic fossil fuel properties, further expand the potential of alternative energy sources. Studies indicate that green diesel and biofuel blends can achieve higher efficiency and lower emissions than conventional diesel, although challenges, such as nitrogen oxide (NOx) emissions, persist [8].
Hybrid fuel blends leverage the advantages of each component, offering a balanced approach to performance, efficiency, and emissions reduction. Studies on ammonia–hydrogen blends have shown substantial reductions in CO2 and particulate matter emissions while maintaining competitive thermal efficiency. Additionally, combining synthetic fuels with biofuels can optimize combustion characteristics and reduce reliance on petroleum-based fuels [9]. These advancements align with global emission reduction targets and contribute to the long-term viability of CI engines.
In response to the urgent need for climate action, many countries have committed to achieving carbon neutrality by the mid-century, prompting a transformation of the transportation and energy sectors. International policies, such as the European Green Deal, the Paris Agreement, and national net-zero frameworks, are driving the decarbonization of internal combustion engine (ICE) systems through stringent emission regulations and support for low-carbon technologies. These global initiatives underscore the necessity of transitioning toward cleaner fuels and advanced combustion strategies in CI engines. Hybrid fuels, with their capacity to reduce greenhouse gases and pollutants, are increasingly viewed as transitional solutions that align with both regulatory mandates and technological feasibility. Therefore, understanding and optimizing hybrid fuel blends is not only a scientific challenge but also a critical step toward meeting international climate goals [10,11,12].
The objective of this paper is to explore the potential of hybrid fuel blends for CI engines, evaluating their impact on engine performance, emissions, and sustainability. By analyzing recent studies and technological developments, this research aims to provide insights into the feasibility of adopting hybrid fuels as a mainstream alternative. The findings will contribute to the ongoing efforts in reducing carbon footprints and promoting sustainable transportation solutions [13]. However, several key research gaps remain, which this review aims to address. These research gaps are as follows:
  • There is a lack of comprehensive reviews that analyze multi-component fuel strategies involving biofuels, synthetic fuels, and additives.
  • The current literature does not sufficiently cover the performance of hybrid fuels under various engine operating conditions, such as different loads, injection pressures, and combustion modes.
  • The effect of metal oxide nano-additives when used in conjunction with hybrid fuels remains inadequately addressed in terms of combustion stability and pollutant reduction.
To systematically address these issues, the paper is structured as follows: Section 2 provides an overview of hybrid fuels; Section 3 discusses the role of biofuels in CI engines; Section 4 explores hydrogen as a CI engine fuel; Section 5 examines ammonia-based combustion strategies; Section 6 reviews the application of synthetic fuels in hybrid blends; Section 7 analyzes the performance and emission characteristics of hybrid fuel systems; Section 8 outlines future research directions; and Section 9 concludes with a summary of findings and their implications for sustainable engine technologies.

2. Overview of Hybrid Fuels

Hybrid fuels are a combination of multiple fuel types, including biofuels, hydrogen, ammonia, and synthetic fuels, to optimize combustion performance and reduce emissions. These fuels can be classified based on their primary components, such as biofuel-hydrogen blends, ammonia–synthetic fuel mixtures, and hybrid combinations incorporating multiple alternative fuels. Each classification serves a distinct purpose, whether to enhance energy density, lower carbon emissions, or improve combustion efficiency in compression ignition engines [14].
The combination of biofuels with hydrogen, ammonia, and synthetic fuels offers several advantages over conventional fuels. Biofuels provide renewable energy with lower carbon intensity, while hydrogen and ammonia contribute to zero-carbon combustion. Synthetic fuels, derived from captured CO2 and renewable energy sources, enhance the fuel’s sustainability profile. Studies indicate that these hybrid blends can significantly reduce greenhouse gas emissions, improve thermal efficiency, and contribute to a more sustainable transportation sector [15].
Different hybrid fuel blends exhibit varying combustion characteristics and efficiency levels. For instance, biofuel–hydrogen blends have been shown to increase flame speed and enhance fuel–air mixing, leading to improved combustion efficiency. Ammonia-based hybrid fuels, while offering carbon-free combustion, require additional ignition enhancement strategies due to their lower reactivity. Synthetic fuel blends, on the other hand, can be engineered to match the properties of conventional fossil fuels, making them a viable drop-in replacement in existing CI engines [16].
Blending can enhance the fuel’s cetane number, which directly correlates with engine performance and emissions [17]. Studies indicate that such blends can reduce particulate matter and NOX emissions compared to traditional diesel [18]. However, challenges related to storage, infrastructure, and hydrogen’s low volumetric energy density must also be considered. Hydrogen storage technologies often require stringent safety measures that can complicate engine retrofitting and regulatory compliance [19].
Ammonia–synthetic fuel blends exhibit characteristics suitable for improving sustainability in CIEs. Ammonia, as a carbon-free fuel, can potentially result in minimal CO2 emissions when blended with synthetic fuels. The combustion of ammonia alone, however, poses challenges, such as lower flame stability and higher NOX emissions, due to high nitrogen concentration during combustion [20]. Research suggests that optimizing the fuel composition can mitigate these adverse effects while maintaining sustainability [21]. For instance, the synergy between ammonia and synthetic fuels can increase energy density and improve combustion stability, making this combination an attractive option for engineering applications [22].
Multi-component mixtures expand fuel formulation by integrating different blends to optimize combustion and emissions performance. For instance, a mixture that incorporates biodiesel and diesel hydrocarbons may enhance fuel stability and reduce the risk of phase separation, a common problem in biofuel utilization [23]. The addition of dispersive additives has been shown to stabilize these mixtures further, enhancing combustion efficiency and reducing emissions [24]. However, the complex nature of multi-component fuels necessitates thorough evaluation of their physical and chemical properties, complicating the regulatory approval processes due to safety and environmental concerns [25].
Regarding energy density, hydrogen has the highest specific energy but a considerably lower volumetric energy density, making it less favorable for certain applications unless effectively stored [20]. While ammonia offers higher volumetric energy density than hydrogen, its energy content still trails behind conventional fossil fuels and necessitates advancements in combustion technology to leverage its potential fully. Multi-component mixtures can bridge these gaps by capitalizing on the strengths of their individual components, often providing a balanced energy profile for a smoother transition while meeting energy demands efficiently [24].
Engine compatibility remains a significant barrier to the widespread adoption of hybrid fuel blends. Conventional engines, particularly diesel engines, may require modifications to utilize blends with higher concentrations of biofuels or ammonia. Changes in viscosity and lubricity, for instance, can adversely affect fuel injection systems and engine materials [17]. Additionally, the complex combustion characteristics of these blends can lead to challenges in achieving optimal combustion timing and performance without compromising engine lifespan. Rigorous testing and validation are critical to ascertain engine performance using these hybrid fuel blends [23].
Regulatory compliance is paramount as nations continue to tighten emissions standards to combat climate change. While blending biocomponents can often provide an immediate reduction in greenhouse gas emissions, the full lifecycle analysis of such fuels becomes complicated [21]. For instance, the sustainability of biofuels is contingent upon their source and production methods, which must be scrutinized to ensure adherence to environmental regulations. Hybrid fuel blends that incorporate renewable components also need to meet specific legislative frameworks to qualify for incentives and avoid penalties associated with fossil fuel usage [18].
To ensure the sustainability of hybrid fuel systems, a comprehensive evaluation must extend beyond tailpipe emissions to include the environmental and socioeconomic impacts of fuel production. For example, waste-derived biofuels—when integrated with low-carbon carriers, like hydrogen—can help reduce overall lifecycle emissions [26]. However, sustainability claims should be critically assessed, especially in cases where biofuel feedstocks compete with food production or contribute to deforestation [21]. A systems-level perspective is essential to balance carbon benefits with resource equity and land-use ethics.
Holistic integration of hybrid fuels into transportation and industrial applications is critical in mitigating reliance on fossil fuels. The synergy between different fuels can create flexible energy solutions capable of adapting to various operational demands and regulatory standards [19]. This makes hybrid fuel blends an essential cog in the machinery of future energy systems, with ongoing research being crucial to define best practices in their development and implementation.
The development of hybrid fuel blends aims to address key challenges, such as fuel storage, combustion optimization, and emissions reduction. Research efforts are focused on improving fuel formulations to enhance ignition properties and thermal efficiency while minimizing the production of harmful pollutants. As the transportation industry moves toward stricter emissions regulations, hybrid fuels represent a promising pathway for maintaining the viability of compression ignition engines in a low-carbon future [27].
Table 1 provides an overview of hybrid fuel components, their production sources, typical blend ratios, and injection strategies employed in compression ignition (CI) engines.

3. Biofuels for CI Engines

With the growing global demand for sustainable energy, biodiesel feedstocks have evolved from first-generation edible vegetable oils to second-generation non-edible oils and waste fats, then to third-generation microalgal resources, and ultimately to fourth-generation microbial factory pathways based on synthetic biology. Although first-generation feedstocks offer good availability and conversion efficiency, their sustainability has been questioned due to competition with food and ecological impacts. Second-generation feedstocks mitigate the “food vs. fuel” conflict by utilizing non-edible and waste resources but are limited by challenges in raw material collection and complex pretreatment processes. Third-generation microalgal feedstocks show great potential due to their high oil yield and carbon capture capacity, but still face technical and economic barriers in terms of cultivation and lipid extraction. The fourth generation, enabled by genetic engineering, allows for the direct synthesis of target fuel molecules, offering both flexibility in raw material sources and controllability over product properties, making it an ideal pathway for future development [28].
Globally, governments are actively promoting the production and application of biodiesel by implementing a range of incentive policies to support the development of renewable energy and the transformation of energy structures. These policies primarily include financial subsidies, tax incentives, mandatory blending ratios, green credit support, and carbon emission trading schemes. For instance, the United States has established the Renewable Fuel Standard (RFS), which mandates minimum blending levels of biodiesel in transportation fuels and provides tax benefits to producers, along with a Renewable Identification Number (RIN) trading mechanism. In the European Union, member states have set specific targets for the share of renewable energy in the transportation sector under the Renewable Energy Directive and have implemented sustainability certification systems for biofuels. Major biodiesel-producing countries, such as Brazil and Argentina, have legislated minimum biodiesel blending mandates and established price subsidy mechanisms to ensure stable domestic market development [29].
Hybridization of biofuels with other alternative energy sources has gained attention as a strategy to overcome their limitations while improving combustion efficiency and emission characteristics. The combination of biofuels with hydrogen has been shown to enhance flame propagation speed and thermal efficiency while reducing particulate matter and carbon emissions. However, hydrogen’s low energy density and storage difficulties present practical challenges for large-scale implementation [30]. Similarly, ammonia, a zero-carbon fuel, can be blended with biodiesel or ethanol to achieve near-complete decarbonization of CI engines. While ammonia’s high ignition temperature and lower combustion efficiency require modifications, such as dual-fuel injection strategies, research suggests that combining ammonia with biofuels can significantly reduce nitrogen oxide emissions without compromising engine performance [31].
Another promising approach to improving biofuel performance is the integration of synthetic fuels engineered to mimic the chemical properties of petroleum-based diesel. Synthetic fuels derived from carbon capture and renewable energy sources offer a cleaner alternative that can be blended with biofuels to optimize fuel combustion and energy output. Studies indicate that synthetic fuel–biodiesel blends not only lower greenhouse gas emissions but also improve fuel atomization and engine longevity, addressing some of the drawbacks of pure biofuels [32]. Additionally, advanced fuel formulations incorporating nano-additives, such as aluminum oxide or carbon nanotubes, have demonstrated improvements in combustion stability, thermal efficiency, and emissions reduction [33].
The continued development and optimization of hybrid biofuels will be critical in achieving long-term sustainability for CI engines, particularly in sectors where full electrification remains challenging. While the combination of biofuels with hydrogen, ammonia, and synthetic fuels presents new opportunities, further research is needed to address technical challenges, such as fuel storage, ignition optimization, and economic feasibility. Policymakers and industry stakeholders must work together to support advancements in fuel blending technologies, improve regulatory frameworks, and promote large-scale adoption of hybrid biofuels to reduce dependency on fossil fuels and meet global emissions reduction targets. Ultimately, hybrid biofuels represent a key transitional solution in the shift toward greener and more sustainable transportation systems.

4. Hydrogen as a CI Engine Fuel

The application of hydrogen in CI engines presents an innovative avenue for achieving carbon-free combustion while leveraging the high thermal efficiency of diesel engines [34,35]. Hydrogen, as a fuel, possesses superior combustion characteristics, such as a high flame speed and a broad flammability range, which enhance the efficiency of the combustion process [36]. However, the absence of carbon in hydrogen combustion results in negligible CO2 emissions, making it an attractive alternative to conventional fossil fuels. One of the key challenges is the autoignition temperature of hydrogen, which is significantly higher than diesel, necessitating the use of advanced ignition strategies or hydrogen–diesel dual-fuel configurations [37,38].
Hydrogen enrichment in diesel engines has been explored as a means to improve combustion efficiency while reducing emissions. Studies indicate that introducing small fractions of hydrogen into the intake air of a diesel engine enhances fuel–air mixing and results in a more complete combustion process, reducing particulate matter (PM) and carbon monoxide (CO) emissions [36,39]. Furthermore, research has shown that hydrogen enrichment in dual-fuel mode contributes to improved thermal efficiency by enhancing flame propagation, especially under lean-burn conditions [40]. However, hydrogen’s high diffusivity and low density pose storage and handling challenges, requiring advanced fuel injection and safety systems [41].
Hydrogen–diesel dual-fuel combustion strategies are gaining traction due to their potential to mitigate NOx emissions while maintaining high engine efficiency. Recent studies highlight that partial replacement of diesel with hydrogen significantly reduces soot formation due to the absence of carbon in hydrogen, thereby improving air quality and reducing the environmental footprint [42,43]. Additionally, experimental investigations demonstrate that using hydrogen as a secondary fuel in CI engines enables better control of combustion phasing and enhances knock resistance, a crucial factor in engine performance [38]. However, challenges, such as pre-ignition due to hydrogen’s low ignition energy and increased NOx formation at higher hydrogen fractions, need to be addressed through optimized injection timing and exhaust gas recirculation (EGR) strategies [44].
Another promising avenue is the blending of hydrogen with biofuels, which combines the advantages of renewable energy sources with the clean-burning properties of hydrogen. Biofuel–hydrogen blends have been shown to reduce overall greenhouse gas emissions while improving combustion efficiency in CI engines [39,40]. Research suggests that combining biodiesel with hydrogen leads to a synergistic effect, reducing PM and unburned hydrocarbon emissions while maintaining thermal efficiency [45]. However, blending hydrogen with biofuels requires careful optimization of injection strategies and combustion chamber design to prevent issues, such as fuel stratification and knocking [38,46].
While hydrogen presents a promising solution for reducing carbon emissions in CI engines, its practical implementation requires addressing safety and storage challenges. Hydrogen’s low energy density per unit volume necessitates high-pressure storage or cryogenic liquefaction, both of which add complexity and costs to fuel infrastructure. Additionally, hydrogen’s propensity for leakage due to its small molecular size raises safety concerns, requiring robust containment and detection systems [40,41,42,43,44,45]. Addressing these technical and infrastructural challenges is crucial to unlocking the full potential of hydrogen-powered CI engines.
A. Madhan Kumar et al. [46] reported that adding 100 ppm of TiO2 nanoparticles to M20 fuel (a blend of 20% Madhuca biodiesel and 80% diesel) and supplying 10 L/min of hydrogen as an auxiliary fuel increased the brake thermal efficiency (BTE) of a diesel engine by 24.23% and reduced the brake specific fuel consumption (BSFC) by 8.32%. In addition, hydrocarbon (HC), carbon monoxide (CO), and smoke emissions were reduced by 13.16%, 19.02%, and 4.04%, respectively, although NOX emissions increased by 12.67%, demonstrating the significant synergistic effect of hydrogen and TiO2 in enhancing engine performance and reducing emissions. G.K. Jhanani et al. [47] introduced 5 L/min of hydrogen into an unmodified diesel engine fueled with a blend of 20% Scenedesmus dimorphus microalgae biodiesel and 80% diesel (SDM20%). Under 3000 rpm conditions, the engine achieved a maximum power output of 5.2 kW (SDM0%H5), and CO and CO2 emissions dropped to 0.11 ppm and 11.9%, respectively, representing reductions of approximately 0.4 ppm and 0.9% compared to neat diesel.
Although NOX emissions increased to 1460 ppm under the SDM20%H5 condition, the results indicate that this fuel combination significantly improves thermal efficiency and promotes cleaner combustion. G.M. Lionus Leo et al. [48] found that introducing 6 L/min of hydrogen into a 50% cashew nutshell oil biodiesel and 50% diesel blend (50CNSO50D) increased the brake thermal efficiency (BTE) of a diesel engine by 4.4% (from 30.4% to 34.7%). Simultaneously, CO, HC, and CO2 emissions were reduced by 5.6%, 22.2%, and 7%, respectively, although NOX emissions rose by 6.3%, indicating that the dual-fuel combination effectively enhances combustion performance while mitigating emissions. Prem Shanker Yadav et al. [49] investigated a B20 blend (20% waste cooking oil biodiesel + 80% diesel) preheated to 60 °C, with hydrogen added at flow rates of 2, 5, 10, and 15 L/min. The highest brake thermal efficiency (BTE) of 36.33% was achieved with 5 L/min hydrogen, representing a 17.18% increase over diesel, while the lowest BSFC of 0.23 kg/kWh showed a 42.26% reduction compared to B20. At full load, the H15B20 condition yielded CO, HC, and CO2 emissions of 0.12%V, 22 ppm, and 4.8%V, respectively, but NOX emissions rose to 2300 ppm—31.97% higher than diesel—demonstrating that low hydrogen flow significantly enhances performance and reduces emissions, whereas high hydrogen flow notably increases NOX.

5. Ammonia as a CI Engine Fuel

Ammonia (NH3) is gaining attention as a carbon-free fuel for CI engines due to its potential for zero-carbon energy cycles and compatibility with existing fuel infrastructure. Its application in CI engines is, however, constrained by its combustion properties, NOx emissions, and storage challenges. To optimize its performance, researchers have investigated its co-combustion with biofuels, diesel, and hydrogen while also exploring innovative fuel injection techniques and catalytic decomposition strategies [50,51].

5.1. Properties of Ammonia and Feasibility as a Fuel

Ammonia is an attractive alternative fuel due to its high hydrogen content (17.6% by mass), ease of liquefaction at moderate pressures (≈10 bar at room temperature), and potential for sustainable production through renewable energy-based electrolysis. However, its lower heating value (18.6 MJ/kg) is significantly lower than diesel (42.5 MJ/kg), which affects engine power output and combustion efficiency. Additionally, ammonia has a high autoignition temperature (651 °C), requiring either high compression ratios or ignition assistance when used in CI engines [51,52,53,54]. These properties necessitate modifications in fuel injection strategies and air–fuel mixing to ensure efficient combustion in CI engines.

5.2. Combustion Characteristics and NOx Formation Challenges

Ammonia combustion in compression ignition (CI) engines presents notable challenges, primarily due to its low flame speed and high ignition energy, which result in incomplete combustion, elevated NOx emissions, and unburned ammonia (ammonia slip). These issues often necessitate aftertreatment systems, such as selective catalytic reduction (SCR), or strategies, like exhaust gas recirculation (EGR), for emissions control. Research suggests that ammonia’s combustion efficiency can be enhanced through dual-fuel operation—blending ammonia with a more reactive fuel—and by optimizing the air–fuel ratio [55,56].
To further understand nitrogen oxide formation in such systems, a spatiotemporal analysis that separates fuel-borne and airborne nitrogen provides valuable insight. This approach reveals that thermal NOx (NO and NO2) forms predominantly at the periphery of the diffusion flame during the later stages of combustion, with its distribution influenced by in-cylinder flow dynamics. In contrast, fuel NOx begins forming early in the combustion process, peaking in regions of intense heat before converting into other nitrogen species. Additionally, N2O, a byproduct of partial ammonia oxidation, appears near the diffusion flame edge and increases in concentration during the late oxidation phase due to cooler temperatures favoring its formation. This underscores the importance of advanced combustion strategies and pollutant control technologies to address the unique challenges of ammonia-fueled CI engines [57].

5.3. Co-Combustion of Ammonia with Biodiesel, Diesel, or Hydrogen

To address the ignition and combustion stability issues of ammonia, researchers have focused on co-combustion strategies with biodiesel, diesel, and hydrogen. Biodiesel-ammonia blends have shown improved ignition characteristics due to biodiesel’s high cetane number, but challenges remain in ensuring proper air–fuel mixing and reducing unburned NH3 emissions. Diesel–ammonia dual-fuel engines, using pilot injections of diesel to ignite ammonia, have demonstrated higher combustion efficiency and reduced CO2 emissions, though NOx control remains a concern [58,59]. Hydrogen–ammonia blends have been particularly promising, as hydrogen’s high flame speed compensates for ammonia’s poor reactivity, improving combustion efficiency and reducing emissions. However, precise control of fuel ratios and injection timing is necessary to avoid knocking and excessive NOx formation [60]. Ammonia has a very low laminar flame speed of approximately 6–7 cm/s under ambient conditions, which requires a high minimum ignition energy and exhibits a narrow flammability limit. Hydrogen, on the other hand, is one of the fuels with the highest flame speed and the widest flammability range, which may lead to autoignition and thermal explosions in engines and gas turbines. However, when hydrogen and ammonia are blended together, these two extreme fuels can complement each other. For stoichiometric mixtures in air under standard conditions, the laminar flame speed of hydrogen is about 1.7 m/s, while that of ammonia is only 0.07 m/s. When the equivalence ratio exceeds 0.5, the premixed combustion of hydrogen can lead to excessive knocking in engines, severely limiting engine power output. This has prompted the blending of ammonia into hydrogen to overcome the challenges faced by pure hydrogen internal combustion engines (H2ICE) [61].
A summary of recent studies comparing various hybrid fuel blends in terms of brake thermal efficiency, power output, fuel consumption and emissions is presented in Table 2, highlighting the performance variations and research focus areas across different fuel combinations.

5.4. Recent Advancements in Ammonia Fuel Technology

Recent advancements in ammonia combustion technology focus on improving fuel injection strategies, enhancing flame stability, and integrating advanced ignition techniques. Plasma-assisted ignition and laser ignition systems have been proposed as potential solutions to ammonia’s high ignition energy requirement, significantly improving combustion stability and efficiency [69]. Additionally, research into ammonia cracking—where a portion of ammonia is decomposed into hydrogen and nitrogen before combustion—has demonstrated promising results in improving ignition characteristics and reducing NOx emissions. Future research is expected to focus on optimizing ammonia’s role in carbon-neutral energy systems, improving its storage and transportation methods, and further exploring hybridization with biofuels and hydrogen [70].
Alternative combustion strategies, such as homogeneous charge compression ignition (HCCI) and premixed charge compression ignition (PCCI), have been explored with ammonia. Pochet et al. [62] developed an HCCI engine using ammonia-hydrogen blends with variable ratios, enabling high compression ratios and low in-cylinder temperatures. They reported that the autoignition temperature of ammonia was around 610 K, reduced to 440 K with hydrogen addition, improving ignition timing and efficiency. They also applied exhaust gas recirculation (EGR) to lower NOx emissions by reducing oxygen availability, though this slightly decreased combustion efficiency [71].
Earlier, Van Blarigan [72] developed a free-piston linear generator running on HCCI with ammonia, achieving efficiencies comparable to hydrogen. He proposed NOx reduction by reacting exhaust gases with ammonia over a zeolite catalyst via the following reactions:
4NO + 4NH3 + O2 → 4H2 + 6H2O
6NO2 + 8NH3 → 7N2 + 12H2O

6. Synthetic Fuels and Their Role in Hybrid Blends

6.1. Overview of Synthetic Fuels

Synthetic fuels, often categorized as Fischer–Tropsch fuels, e-fuels, and power-to-liquid (PtL) fuels, are gaining significant attention as potential alternatives to conventional fossil fuels in internal combustion engines (ICEs). Fischer–Tropsch synthesis (FTS) produces liquid hydrocarbons from syngas (a mixture of CO and H2), derived from biomass, natural gas, or coal, offering a pathway to cleaner fuel production [73]. E-fuels, or electrofuels, rely on renewable electricity to produce hydrogen via water electrolysis, which is then combined with CO2 through catalytic synthesis to generate hydrocarbons [74]. PtL fuels, a subset of e-fuels, involve the conversion of captured CO2 and green hydrogen into synthetic hydrocarbons through certain processes, such as methanol-to-gasoline (MTG) synthesis [75].
Methanol, a key intermediate in many synthetic fuel pathways, has also been employed directly in internal combustion engines through dual-fuel operation with diesel. This configuration is often achieved by fumigating methanol into the intake manifold, which allows for minimal engine modification [76]. The methanol–air mixture is then ignited by the autoignition of directly injected diesel near top dead center [77]. While this dual-fuel strategy offers cleaner combustion characteristics, it is associated with significantly higher NO2 emissions and a greater NO2-to-NOx (NO and NO2) ratio compared to traditional diesel engines [78]. In conventional diesel operation, NO2 usually contributes about 5% of total NOx emissions at medium to high loads and can rise to 20–30% at low loads, even though the NO2/NOx ratio is still lower than that of spark ignition engines (typically under 2%). The observed surge in NO2 emissions with methanol–diesel dual-fuel engines is a unique phenomenon not typically found in standard compression ignition systems [79]. This increase is particularly concerning due to the severe environmental and health impacts associated with NO2, highlighting the importance of further investigation into combustion chemistry and emission control in synthetic and hybrid fuel applications.
These synthetic fuels provide a drop-in alternative to conventional petroleum-based fuels, enabling existing ICE infrastructure to operate with reduced carbon footprints.

6.2. Production Processes and Sustainability Aspects

The sustainability of synthetic fuels is a critical factor determining their viability. The Fischer–Tropsch process, when integrated with biomass gasification, can achieve near-carbon-neutral fuel production, provided sustainable biomass feedstocks are utilized [80]. Electrofuels depend heavily on the availability of low-carbon electricity; the environmental benefits of e-fuels diminish if fossil-based electricity is used for electrolysis [74]. PtL fuel production typically involves CO2 capture technologies, such as direct air capture (DAC) or industrial CO2 sequestration, which can significantly mitigate greenhouse gas emissions [81]. Nevertheless, challenges, such as high energy input, economic viability, and efficiency losses in conversion steps remain substantial barriers to widespread adoption.

6.3. Application of Synthetic Fuel Blends in CI Engines

CI engines can benefit from synthetic fuel blends due to their high cetane numbers and superior combustion properties. Fischer–Tropsch fuels exhibit low sulfur and aromatic contents, leading to reduced particulate matter (PM) and NOx emissions [73]. Blends of synthetic diesel with biodiesel or fossil-based diesel have demonstrated enhanced combustion efficiency and lower emissions in experimental studies [82]. E-fuels, particularly synthetic diesel produced via PtL processes, offer excellent performance in CI engines while enabling carbon neutrality if renewable energy sources are used [81]. Studies have indicated that synthetic fuel blends can improve ignition characteristics and engine efficiency while minimizing fuel system modifications.

6.4. Advantages over Conventional Fuels

Synthetic fuels provide several advantages over conventional petroleum-based fuels, particularly in terms of environmental impact and fuel properties. Their near-zero sulfur content reduces SOx emissions, contributing to improved air quality and reduced acid rain formation [83,84]. The high cetane number of Fischer–Tropsch fuels enhances combustion efficiency, leading to reduced fuel consumption and lower CO2 emissions [85]. Moreover, synthetic fuels are compatible with existing fuel infrastructure, allowing for seamless integration into current transportation and industrial applications without significant modifications [86]. The ability to produce synthetic fuels from renewable sources also enhances energy security and reduces reliance on geopolitically unstable fossil fuel supplies [87].
However, economic viability remains a major barrier to widespread implementation. The current production cost of renewable synthetic fuels is significantly higher than that of conventional fossil fuels, mainly due to the energy-intensive processes involved and the limited scale of production facilities. As of recent estimates, the cost of synthetic fuels produced via power-to-liquid or biomass-to-liquid routes ranges between USD 3 and USD 6 per liter, compared to approximately USD 0.5 to USD 1 per liter for fossil-derived diesel or gasoline. Nevertheless, with continued technological advancements, policy incentives, and economies of scale, the price of renewable fuels is projected to decline substantially over the next decade. Several forecasts suggest that synthetic fuel costs could become competitive with fossil fuels by the early 2030s, particularly in regions with abundant renewable electricity and supportive regulatory frameworks. This underscores the importance of sustained investment in CO2 capture infrastructure and process optimization as key priorities for future research and industrial deployment [88,89].

7. Performance and Emission Characteristics of Hybrid Fuels

Hybrid fuel blends, consisting of combinations of synthetic fuels, biofuels, and conventional fossil fuels, significantly influence key engine performance metrics, such as brake thermal efficiency (BTE), brake specific fuel consumption (BSFC), and power output. Several studies have demonstrated that blending biodiesel or synthetic fuels with diesel improves BTE due to better combustion characteristics, enhanced cetane number, and improved spray atomization [90,91]. For instance, blends of Fischer–Tropsch (FT) diesel and biodiesel have exhibited higher BTE compared to pure diesel, primarily due to their superior oxidation stability and lower aromatic content [92,93].
Regarding BSFC, hybrid fuels often lead to a slight increase due to their lower energy density compared to petroleum diesel. However, oxygenated fuels, such as biodiesel or alcohol-based blends, can compensate for this by enhancing combustion efficiency [90,94]. Additionally, synthetic fuels derived from power-to-liquid (PtL) processes have been shown to improve combustion efficiency, thereby reducing BSFC [95]. Power output variations depend on the blend composition; for example, FT diesel blends with conventional diesel generally maintain similar power outputs, whereas biodiesel blends might show a slight reduction due to lower calorific value [96].
Hybrid fuel blends can significantly alter the emission profiles of compression ignition engines. One of the major concerns with biofuels and synthetic fuels is their impact on NOx emissions. Studies indicate that oxygenated fuels, such as biodiesel or alcohol–diesel blends, tend to increase NOX emissions due to higher combustion temperatures and faster reaction kinetics [97]. However, adding FT diesel or PtL fuels can mitigate this effect by lowering the flame temperature and improving air–fuel mixing [98].
CO2 emission reduction is one of the key advantages of hybrid fuel blends. Fuels produced via PtL processes, when combined with biofuels or synthetic fuels, result in a lower net carbon footprint since CO2 is captured during production [95,99]. Additionally, the use of waste-based biofuels in hybrid blends further enhances carbon neutrality [100].
PM emissions are significantly reduced with synthetic fuels due to their near-zero sulfur and aromatic content. Fischer–Tropsch diesel blends have shown a drastic reduction in PM emissions compared to fossil diesel, contributing to cleaner combustion [93]. Additionally, oxygenated biofuels promote complete combustion, further lowering PM formation [101].
Table 3 presents a comparative overview of hybrid biofuel strategies for CI engines, highlighting their performance gains, emission impacts, and key challenges. These blends—combining biofuels with hydrogen, ammonia, or synthetic fuels—demonstrate improved efficiency and reduced emissions but also pose technical and economic hurdles.

8. Future Research Directions

To advance the application of hybrid fuels in compression ignition (CI) engines, future research should focus on the following key areas:
Fuel optimization: Developing optimal blending strategies for hybrid fuels to improve combustion stability, thermal efficiency, and emissions performance remains a critical challenge. This includes determining ideal ratios for components, like biofuels, ammonia, hydrogen, and synthetic fuels.
Combustion control: Advanced injection and ignition strategies (e.g., dual-fuel or split injection) are needed to manage the diverse combustion properties of hybrid fuels, especially those with low reactivity, like ammonia.
Emissions mitigation: Tailored aftertreatment systems, such as SCR and EGR, must be developed to address increased NOx or unburned ammonia emissions associated with certain blends.
Economic viability: Cost-effective production, storage, and engine adaptation methods are essential for large-scale implementation. Research should also include lifecycle assessments to evaluate environmental and economic impacts.
Addressing these challenges will be crucial for the practical deployment of hybrid fuels and for meeting future decarbonization goals in the transportation sector.

9. Conclusions

Hybrid fuel technologies integrating biofuels, hydrogen, ammonia, and synthetic fuels present a promising pathway for decarbonizing compression ignition (CI) engines while maintaining high performance and operational flexibility. The findings from recent studies and technological advancements underscore the potential of these blends to significantly reduce greenhouse gas emissions, improve combustion efficiency, and contribute to global sustainability goals.
Hybrid fuel blends leverage the complementary properties of biofuels, hydrogen, ammonia, and synthetic fuels to enhance combustion efficiency and reduce harmful emissions. Biofuels serve as a renewable and engine-compatible foundation, making them ideal for blending with other alternative fuels to lower carbon intensity. Hydrogen offers clean, high-efficiency combustion but requires advanced storage and injection systems for safe and effective use in CI engines. Ammonia is a carbon-free fuel with great potential, especially when co-fired with biodiesel or hydrogen to improve ignition and reduce CO2 emissions. Synthetic fuels can be engineered for high performance and are compatible with current diesel engines, offering a practical drop-in solution for decarbonization.
Advanced combustion techniques, such as homogeneous charge compression ignition (HCCI) and premixed charge compression ignition (PCCI), when combined with hybrid fuels, can significantly reduce NOx and particulate emissions while improving thermal efficiency. Despite the advantages, several technical and infrastructural challenges remain. These include the need for tailored fuel additives, optimized injection timing strategies, and robust safety systems. Nonetheless, continued innovation in engine design and government support through regulatory incentives can accelerate the deployment of hybrid fuel technologies.
Hybrid fuel blends represent a transitional yet powerful solution toward sustainable and low-carbon CI engine operation. Continued interdisciplinary research, pilot-scale demonstrations, and the establishment of supportive policy frameworks are essential to move hybrid fuel technologies from laboratory-scale investigations to real-world implementation. As electrification continues to expand in parallel, hybrid fuels can ensure that CI engines remain an integral part of the energy transition, particularly in hard-to-electrify sectors, such as heavy-duty transportation, agriculture, and marine applications.

Author Contributions

Conceptualization, R.K.; methodology, R.K.; software, R.K.; validation, R.K. and H.M.C.; formal analysis, R.K.; investigation, R.K.; resources, H.M.C.; data curation, R.K.; writing—original draft preparation, R.K.; writing—review and editing, R.K. and H.M.C.; visualization, R.K.; supervision, H.M.C.; project administration, H.M.C.; funding acquisition, H.M.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Khujamberdiev, R.; Cho, H.M. Synthesis and Characterization of Nanoparticles in Transforming Biodiesel into a Sustainable Fuel. Molecules 2025, 30, 1352. [Google Scholar] [CrossRef] [PubMed]
  2. Khujamberdiev, R.; Cho, H. Impact of Biodiesel Blending on Emission Characteristics of One-Cylinder Engine Using Waste Swine Oil. Energies 2023, 16, 5489. [Google Scholar] [CrossRef]
  3. Ai, W.; Cho, H.M. Predictive Models for Biodiesel Performance and Emission Characteristics in Diesel Engines: A Review. Energies 2024, 17, 4805. [Google Scholar] [CrossRef]
  4. Khujamberdiev, R.; Cho, H.M. Artificial Intelligence in Automotives: ANNs’ Impact on Biodiesel Engine Performance and Emissions. Energies 2025, 18, 438. [Google Scholar] [CrossRef]
  5. Pranta, M.H.; Cho, H.M. A Comprehensive Review of the Evolution of Biodiesel Production Technologies. Energy Convers. Manag. 2025, 328, 119623. [Google Scholar] [CrossRef]
  6. Zheng, F.; Cho, H.M. The Effect of Different Mixing Proportions and Different Operating Conditions of Biodiesel Blended Fuel on Emissions and Performance of Compression Ignition Engines. Energies 2024, 17, 344. [Google Scholar] [CrossRef]
  7. Singh, D.K.; Sarma, A.K.; Sandhu, S.S. A Comprehensive Experimental Investigation of Green Diesel as a Fuel for CI Engines. Int. J. Green Energy 2019, 16, 1152–1164. [Google Scholar] [CrossRef]
  8. Allahyari, S.; Mohammadkhani, F.; Yari, M. Combustion and Exergy Investigation of a Compression Ignition Engine Fueled with Ethanol and Methanol Blends. Environ. Eng. Sci. 2021, 9, 159–172. [Google Scholar] [CrossRef]
  9. Tamadonfar, P.; Karimkashi, S.; Kaario, O.T.; Vuorinen, V. A Numerical Study on Premixed Turbulent Planar Ammonia/Air and Ammonia/Hydrogen/Air Flames: An Analysis on Flame Displacement Speed and Burning Velocity. Flow Turbul. Combust. 2023, 111, 717–741. [Google Scholar] [CrossRef]
  10. European Commission. A European Green Deal; European Commission: Brussels, Belgium, 2019; Available online: https://ec.europa.eu/info/strategy/priorities-2019-2024/european-green-deal_en (accessed on 10 May 2025).
  11. United Nations Framework Convention on Climate Change (UNFCCC). The Paris Agreement; UNFCCC: Bonn, Germany, 2015; Available online: https://unfccc.int/process-and-meetings/the-paris-agreement (accessed on 10 May 2025).
  12. International Energy Agency (IEA). Net Zero by 2050: A Roadmap for the Global Energy Sector; IEA: Paris, France, 2021; Available online: https://www.iea.org/reports/net-zero-by-2050 (accessed on 10 May 2025).
  13. Akay, G. Hydrogen, Ammonia and Symbiotic/Smart Fertilizer Production Using Renewable Feedstock and CO2 Utilization through Catalytic Processes and Nonthermal Plasma with Novel Catalysts and In Situ Reactive Separation: A Roadmap for Sustainable and Innovation-Based Technology. Catalysts 2023, 13, 1287. [Google Scholar] [CrossRef]
  14. Martins, J.; Brito, F.P. Alternative Fuels for Internal Combustion Engines. Energies 2020, 13, 4086. [Google Scholar] [CrossRef]
  15. Mansoori, G.A.; Agyarko, L.B.; Estévez, L.A.; Fallahi, B.; Gladyshev, G.D.; Santos, R.G.; Niaki, S.R.; Perivsi’c, O.; Sillanpaa, M.; Tumba, K.; et al. Fuels of the Future for Renewable Energy Sources (Ammonia, Biofuels, Hydrogen). arXiv 2021, arXiv:2102.00439. [Google Scholar]
  16. Valdmanis, G.; Drobins, M.; Bažbauers, G. Use of Synthetic Fuels Derived from Green Hydrogen and CO2 in Heavy-duty and Long-range Transport: The Case of Latvia. In Proceedings of the CONECT. International Scientific Conference of Environmental and Climate Technologies, Riga, Latvia, 10–12 May 2023. [Google Scholar] [CrossRef]
  17. Nastasi, B. Renewable energy generation and integration insustainable buildings—A focus on eco-fuels. Sustain. Build. 2016, 1, 2. [Google Scholar] [CrossRef]
  18. Jiang, L. Performance comparison between traditional internal combustion engines and hybrid powertrains. Highlights Sci. Eng. Technol. 2024, 114, 129–135. [Google Scholar] [CrossRef]
  19. Melo, S.P.; Barke, A.; Cerdas, F.; Thies, C.; Mennenga, M.; Spengler, T.S.; Herrmann, C. Sustainability assessment and engineering of emerging aircraft technologies—Challenges, methods and tools. Sustainability 2020, 12, 5663. [Google Scholar] [CrossRef]
  20. Biçer, Y. Thermodynamic analysis of a renewable energy-driven electric vehicle charging station with on-site electricity generation from hydrogen and ammonia fuel cells. Int. J. Automot. Sci. Technol. 2020, 4, 223–233. [Google Scholar] [CrossRef]
  21. Hezam, I.M.; Mishra, A.R.; Rani, P.; Cavallaro, F.; Saha, A.; Ali, J.; Strielkowski, W.; Štreimikienė, D. A hybrid intuitionistic fuzzy-merec-rs-dnma method for assessing the alternative fuel vehicles with sustainability perspectives. Sustainability 2022, 14, 5463. [Google Scholar] [CrossRef]
  22. Zhao, J.; Wang, F.; Ruan, Q.; Wu, Y.; Zhang, B.; Lü, Y. Hybrid energy storage systems for fast-developing renewable energy plants. J. Phys. Energy 2024, 6, 042003. [Google Scholar] [CrossRef]
  23. Al-Esawi, N.; Qubeissi, M.A.; Whitaker, R.; Сажин, С. Blended e85–diesel fuel droplet heating and evaporation. Energy Amp; Fuels 2019, 33, 2477–2488. [Google Scholar] [CrossRef]
  24. Wang, Z.; Paulauskienė, T.; Uebe, J.; Bučas, M. Characterization of biomethanol–biodiesel–diesel blends as alternative fuel for marine applications. J. Mar. Sci. Eng. 2020, 8, 730. [Google Scholar] [CrossRef]
  25. Ackermann, P.; Braun, K.E.; Burkardt, P.; Heger, S.; König, A.; Morsch, P.; Lehrheuer, B.; Surger, M.; Völker, S.; Blank, L.M.; et al. Designed to be green, economic, and efficient: A ketone-ester-alcohol-alkane blend for future spark-ignition engines. ChemSusChem 2021, 14, 5254–5264. [Google Scholar] [CrossRef] [PubMed]
  26. Daniel, J.; Alvarez, A.A.S.; Heesen, P.T.; Lehrheuer, B.; Pischinger, S.; Hollert, H.; Roß-Nickoll, M.; Du, M. Air-liquid interface exposure of a549 human lung cells to characterize the hazard potential of a gaseous bio-hybrid fuel blend. PLoS ONE 2024, 19, e0300772. [Google Scholar] [CrossRef]
  27. Zheng, F.; Cho, H.M. Study on Biodiesel Production: Feedstock Evolution, Catalyst Selection, and Influencing Factors Analysis. Energies 2025, 18, 2533. [Google Scholar] [CrossRef]
  28. Atabani, A.E.; Silitonga, A.S.; Badruddin, I.A.; Mahlia, T.M.I.; Masjuki, H.H.; Mekhilef, S. A comprehensive review on biodiesel as an alternative energy resource and its characteristics. Renew. Sustain. Energy Rev. 2012, 16, 2070–2093. [Google Scholar] [CrossRef]
  29. Soloiu, V.; Weaver, A.; Smith, R.; Rowell, A.; Mcafee, J.; Willis, J. Combustion Characteristics of Low DCN Synthetic Aviation Fuel, IPK, in a High Compression Ignition Indirect Injection Research Engine; SAE Technical Paper Series; SAE International: Warrendale, PA, USA, 2023. [Google Scholar] [CrossRef]
  30. Weldeslase, M.G.; Benti, N.E.; Desta, M.A.; Mekonnen, Y.S. Maximizing biodiesel production from waste cooking oil with lime-based zinc-doped CaO using response surface methodology. Sci. Rep. 2023, 13, 4430. [Google Scholar] [CrossRef]
  31. Peddamangari, C.; Kilari, N.; Shanmugasundaram, A. A Review on Synthesis, Combustion, Performance and Emissions of Compression Ignition Engine Fueled by Waste Cooking Oil Based Biodiesel. Egypt. J. Pet. 2024, 33, 10. [Google Scholar] [CrossRef]
  32. Singh, R.; Porwal, R.K.; Verma, V. Effects of CNT Nanoparticles’ on the Performance and Emission Study of CI Engines Utilizing a Combination of Diesel and Waste Cooking Oil Biodiesel. J. Mines Met. Fuels 2024, 72, 369–376. [Google Scholar] [CrossRef]
  33. Fasogbon, S.K. Nano-additive blends examination of performance and emission profile of CI engines fuelled with waste cooking oil based-biodiesel. J. Therm. Eng. 2025, 11, 1–15. [Google Scholar] [CrossRef]
  34. Khedr, A.M.; El-Adawy, M.; Ismael, M.A.; Qador, A.; Abdelhafez, A.; Ben-Mansour, R.; Nemitallah, M.A. Recent Fuel-Based Advancements of Internal Combustion Engines: Status and Perspectives. Energy Fuels 2025, 39, 5099–5132. [Google Scholar] [CrossRef]
  35. Shahid, M.I.; Rao, A.; Farhan, M.; Liu, Y.; Salam, H.A.; Chen, T.; Ma, F. Hydrogen production techniques and use of hydrogen in internal combustion engine: A comprehensive review. Fuel 2024, 378, 132769. [Google Scholar] [CrossRef]
  36. Sánchez, A.L.; Williams, F.A. Recent advances in understanding of flammability characteristics of hydrogen. Prog. Energy Combust. Sci. 2014, 41, 1–55. [Google Scholar] [CrossRef]
  37. Shadidi, B.; Najafi, G.; Yusaf, T. A review of hydrogen as a fuel in internal combustion engines. Energies 2021, 14, 6209. [Google Scholar] [CrossRef]
  38. Stępień, Z. A comprehensive overview of hydrogen-fueled internal combustion engines: Achievements and future challenges. Energies 2021, 14, 6504. [Google Scholar] [CrossRef]
  39. Chaichan, M.T. The effects of hydrogen addition to diesel fuel on the emitted particulate matters. Int. J. Sci. Eng. Res. 2015, 6, 1081–1087. [Google Scholar]
  40. Zhou, J.H.; Cheung, C.S.; Zhao, W.Z.; Leung, C.W. Diesel–hydrogen dual-fuel combustion and its impact on unregulated gaseous emissions and particulate emissions under different engine loads and engine speeds. Energy 2016, 94, 110–123. [Google Scholar] [CrossRef]
  41. Li, H.; Cao, X.; Liu, Y.; Shao, Y.; Nan, Z.; Teng, L.; Bian, J. Safety of hydrogen storage and transportation: An overview on mechanisms, techniques, and challenges. Energy Rep. 2022, 8, 6258–6269. [Google Scholar] [CrossRef]
  42. Rueda-Vázquez, J.M.; Serrano, J.; Pinzi, S.; Jiménez-Espadafor, F.J.; Dorado, M.P. A Review of the Use of Hydrogen in Compression Ignition Engines with Dual-Fuel Technology and Techniques for Reducing NOx Emissions. Sustainability 2024, 16, 3462. [Google Scholar] [CrossRef]
  43. Wright, M.L.; Lewis, A.C. Decarbonisation of heavy-duty diesel engines using hydrogen fuel: A review of the potential impact on NO x emissions. Environ. Sci. Atmos. 2022, 2, 852–866. [Google Scholar] [CrossRef]
  44. Singh, P.; Sahoo, S.; Kumbhakarna, N.; Singh, P. Hydrogen-Fueled Spark Ignition Engines: Understanding NOx Formation and Mitigation Through Engine Design Innovations. In Ammonia and Hydrogen for Green Energy Transition; Springer Nature: Singapore, 2024; pp. 371–392. [Google Scholar]
  45. Banerjee, R.; Roy, S.; Bose, P.K. Hydrogen-EGR synergy as a promising pathway to meet the PM–NOx–BSFC trade-off contingencies of the diesel engine: A comprehensive review. Int. J. Hydrogen Energy 2015, 40, 12824–12847. [Google Scholar] [CrossRef]
  46. Kumar, A.M.; Nathan, K.S.; Manickam, S.; Gupta, D.; Vikneswaran, M.; Dhamodaran, G.; Seetharaman, S.; Thiagarajan, S.; Al-Ansari, M.M. Effect of TiO2 nanoparticles and hydrogen on the combustion, performance, and emissions of madhuca biodiesel in a diesel engine. Int. J. Hydrogen Energy 2025. [Google Scholar] [CrossRef]
  47. Jhanani, G.K.; Salmen, S.H.; Al Obaid, S.; Mathimani, T. Performance and emission analysis of Scenedesmus dimorphus-based biodiesel with hydrogen as fuel in an unmodified compression ignition engine. Int. J. Hydrog. Energy 2025, 99, 1132–1138. [Google Scholar] [CrossRef]
  48. Leo, G.L.; Jayabal, R.; Kathapillai, A.; Sekar, S. Performance and emissions optimization of a dual-fuel diesel engine powered by cashew nut shell oil biodiesel/hydrogen gas using response surface methodology. Fuel 2025, 384, 133960. [Google Scholar]
  49. Shanker Yadav, P.; Said, Z.; Gautam, R.; Caliskan, H.; Wu, H. Impact of hydrogen induction on atomization combustion performance and emissions in diesel engines fueled with heated biodiesel blends. Energy 2024, 313, 134026. [Google Scholar] [CrossRef]
  50. Zbikowski, M.; Teodorczyk, A. Machine Learning for Internal Combustion Engine Optimization with Hydrogen-Blended Fuels: A Literature Review. Energies 2025, 18, 1391. [Google Scholar] [CrossRef]
  51. Jafar, U.; Nuhu, U.; Khan, W.U.; Hossain, M.M. A review on green ammonia as a potential CO2 free fuel. Int. J. Hydrogen Energy 2024, 71, 857–876. [Google Scholar] [CrossRef]
  52. Tawalbeh, M.; Murtaza, S.Z.; Al-Othman, A.; Alami, A.H.; Singh, K.; Olabi, A.G. Ammonia: A versatile candidate for the use in energy storage systems. Renew. Energy 2022, 194, 955–977. [Google Scholar] [CrossRef]
  53. Wang, B.; Yang, C.; Wang, H.; Hu, D.; Wang, Y. Effect of Diesel-Ignited Ammonia/Hydrogen mixture fuel combustion on engine combustion and emission performance. Fuel 2023, 331, 125865. [Google Scholar] [CrossRef]
  54. Ji, M.; Wu, Z.; Ferrari, A.; Vento, O.; Shang, Q.; Zhang, G.; Li, L. Experimental investigation on the turbulence flame autoignition characteristics of ammonia in a high-temperature co-flow. Fuel 2024, 372, 132216. [Google Scholar] [CrossRef]
  55. Ditaranto, M.; Saanum, I. Experimental study on the effect of pressure on single and two stage combustion of decomposed ammonia (NH3 H2 N2) blends over a swirl stabilized burner. Combust. Flame 2024, 262, 113368. [Google Scholar] [CrossRef]
  56. Bobi, S.; Khaleel Rahman, R.; Garai, P.; Zamora, D.S.; Ahmed, M.; Vasu, S. Analysis of NH3 slip and NOx formation for Ammonia powered aviation engines. In Proceedings of the AIAA SCITECH 2024 Forum, Orlando, FL, USA, 8–12 January 2024; p. 0965. [Google Scholar]
  57. Yang, R.; Liu, J.; Liu, Z.; Liu, J. Applying separate treatment of fuel-and air-borne nitrogen to enhance understanding of in-cylinder nitrogen-based pollutants formation and evolution in ammonia-diesel dual fuel engines. Sustain. Energy Technol. Assess. 2024, 69, 103910. [Google Scholar] [CrossRef]
  58. Reiter, A.J.; Kong, S.C. Demonstration of compression-ignition engine combustion using ammonia in reducing greenhouse gas emissions. Energy Fuels 2008, 22, 2963–2971. [Google Scholar] [CrossRef]
  59. Nadimi, E.; Przybyła, G.; Lewandowski, M.T.; Adamczyk, W. Effects of ammonia on combustion, emissions, and performance of the ammonia/diesel dual-fuel compression ignition engine. J. Energy Inst. 2023, 107, 101158. [Google Scholar] [CrossRef]
  60. Ge, H.; Bakir, A.H.; Zhao, P. Knock mitigation and power enhancement of hydrogen spark-ignition engine through ammonia blending. Machines 2023, 11, 651. [Google Scholar] [CrossRef]
  61. Lee, B.; Ge, H.; Parameswaran, S.; Zhao, P.; Rochester, M.I. CFD Simulation of a Premixed Spark Ignition Hydrogen Engine. In Proceedings of the Internal Combustion Engine Division Fall Technical Conference, American Society of Mechanical Engineers, Chicago, IL, USA, 20–23 October 2019. [Google Scholar]
  62. Pochet, M.; Truedsson, I.; Foucher, F.; Jeanmart, H.; Contino, F. Ammonia-Hydrogen Blends in Homogeneous-Charge Compression-Ignition Engine; SAE Technical Paper 2017-24-0087; SAE International: Warrendale, PA, USA, 2017. [Google Scholar] [CrossRef]
  63. Pochet, M.; Jeanmart, H.; Contino, F. A 22:1 Compression Ratio Ammonia-Hydrogen HCCI Engine: Combustion, Load, and Emission Performances. Front. Mech. Eng. 2020, 6, 43. [Google Scholar] [CrossRef]
  64. Manigandan, S.; Ryu, J.I.; Kumar, T.P.; Elgendi, M. Hydrogen and Ammonia as a Primary Fuel– A Critical Review of Production Technologies, Diesel Engine Applications, and Challenges. Fuel 2023, 352, 129100. [Google Scholar] [CrossRef]
  65. Pham, Q.; Park, S.; Agarwal, A.K.; Park, S. Review of Dual-Fuel Combustion in the Compression-Ignition Engine: Spray, Combustion, and Emission. Energy 2022, 250, 123778. [Google Scholar] [CrossRef]
  66. Abdulwahid, M.A.; Al-Dawody, M.F.; Al-Obeidi, W.; Al-Farhany, K.; Mohamed, M.H.; Jamshed, W.; Eid, M.R.; Alqahtani, H. Assessment of Diesel Engine Thermo-Characteristics Working with Hybrid Fuel Blends. Numer. Heat Transf. Part A Appl. 2023, 84, 659–674. [Google Scholar] [CrossRef]
  67. Yousefi, A.; Guo, H.; Dev, S.; Liko, B.; Lafrance, S. Effects of Ammonia Energy Fraction and Diesel Injection Timing on Combustion and Emissions of an Ammonia/Diesel Dual-Fuel Engine. Soc. Sci. Res. Netw. 2021, 314, 122723. [Google Scholar] [CrossRef]
  68. Jayabal, R.; Leo, G.L.; Das, M.C.; Sekar, S.; Arivazhagan, S. Arivazhagan. Impact of Ammonia Energy Fraction on Improving Thermal Efficiency and Emissions of Ammonia/Biodiesel in Dual Fuel Diesel Engine. Chem. Eng. Res. Des. 2024, 188, 1398–1410. [Google Scholar]
  69. Shah, Z.A.; Mehdi, G.; Congedo, P.M.; Mazzeo, D.; De Giorgi, M.G. A review of recent studies and emerging trends in plasma-assisted combustion of ammonia as an effective hydrogen carrier. Int. J. Hydrogen Energy 2024, 51, 354–374. [Google Scholar] [CrossRef]
  70. Alboshmina, N. Ammonia Cracking with Heat Transfer Improvement Technology. Doctoral Dissertation, Cardiff University, Cardiff, Wales, 2019. [Google Scholar]
  71. Dimitriou, P.; Javaid, R. A review of ammonia as a compression ignition engine fuel. Int. J. Hydrogen Energy 2020, 45, 7098–7118. [Google Scholar] [CrossRef]
  72. Van Blarigan, P. Advanced internal combustion engine research. In Doe Hydrogen Program Review Nrel-Cp-570-28890; Sandia National Laboratories: Livermore, CA, USA, 2000; pp. 1–19. [Google Scholar]
  73. Jahangiri, H.; Bennett, J.; Mahjoubi, P.; Wilson, K.; Gu, S. A review of advanced catalyst development for Fischer–Tropsch synthesis of hydrocarbons from biomass derived syn-gas. Catal. Sci. Technol. 2014, 4, 2210–2229. [Google Scholar] [CrossRef]
  74. Dell’Aversano, S.; Villante, C.; Gallucci, K.; Vanga, G.; Di Giuliano, A. E-fuels: A comprehensive review of the most promising technological alternatives towards an energy transition. Energies 2024, 17, 3995. [Google Scholar] [CrossRef]
  75. Ravi, S.S.; Mazumder, J.; Sun, J.; Brace, C.; Turner, J.W. Techno-Economic assessment of synthetic E-Fuels derived from atmospheric CO2 and green hydrogen. Energy Convers. Manag. 2023, 291, 117271. [Google Scholar] [CrossRef]
  76. Duraisamy, G.; Rangasamy, M.; Govindan, N. A comparative study on methanol/diesel and methanol/PODE dual fuel RCCI combustion in an automotive diesel engine. Renew. Energy 2020, 145, 542–556. [Google Scholar] [CrossRef]
  77. Wang, L.J.; Song, R.Z.; Zou, H.B.; Liu, S.H.; Zhou, L.B. Study on combustion characteristics of a methanol—Diesel dual-fuel compression ignition engine. Proc. Inst. Mech. Eng. Part D J. Automob. Eng. 2008, 222, 619–627. [Google Scholar] [CrossRef]
  78. Lu, H.; Yao, A.; Yao, C.; Chen, C.; Wang, B. An investigation on the characteristics of and influence factors for NO2 formation in diesel/methanol dual fuel engine. Fuel 2019, 235, 617–626. [Google Scholar] [CrossRef]
  79. Yang, R.; Shang, T.; Li, L.; Liu, J.; Xie, T.; Liu, Z.; Liu, J. The mechanism of the increased ratio of nitrogen dioxide to nitrogen oxides in methanol/diesel dual fuel engines. Energy 2024, 312, 133701. [Google Scholar] [CrossRef]
  80. Čuček, L.; Martín, M.; Grossmann, I.E.; Kravanja, Z. Multi-period synthesis of optimally integrated biomass and bioenergy supply network. Comput. Chem. Eng. 2014, 66, 57–70. [Google Scholar] [CrossRef]
  81. Desport, L.; Selosse, S. An overview of CO2 capture and utilization in energy models. Resour. Conserv. Recycl. 2022, 180, 106150. [Google Scholar] [CrossRef]
  82. Yesilyurt, M.K. A detailed investigation on the performance, combustion, and exhaust emission characteristics of a diesel engine running on the blend of diesel fuel, biodiesel and 1-heptanol (C7 alcohol) as a next-generation higher alcohol. Fuel 2020, 275, 117893. [Google Scholar] [CrossRef]
  83. Ram, V.; Salkuti, S.R. An overview of major synthetic fuels. Energies 2023, 16, 2834. [Google Scholar] [CrossRef]
  84. Sadeq, A.M. Alternative Fuels for Sustainable Combustion; 2024; Available online: https://www.researchgate.net/publication/382547111_Alternative_Fuels_for_Sustainable_Combustion#fullTextFileContent (accessed on 27 April 2025)ISBN 9798990783690.
  85. Yuan, H.; Tsukuda, T.; Yang, Y.; Shibata, G.; Kobashi, Y.; Ogawa, H. Effects of chemical compositions and cetane number of Fischer–Tropsch fuels on diesel engine performance. Energies 2022, 15, 4047. [Google Scholar] [CrossRef]
  86. Janaki, S.T.; Madheswaran, D.K.; Naresh, G.; Praveenkumar, T. Beyond fossil: The synthetic fuel surge for a green-energy resurgence. Clean Energy 2024, 8, 1–19. [Google Scholar] [CrossRef]
  87. Hoffart, F.M.; D’Orazio, P.; Kemfert, C. Geopolitical and climate risks threaten financial stability and energy transitions. Energy 2022, 2004, 2965. [Google Scholar]
  88. Pavlenko, N.; Searle, S.; Christensen, A. The Cost of Supporting Alternative Jet Fuels in the European Union; International Council on Clean Transportation (ICCT): Washington, DC, USA, 2019; Available online: https://theicct.org/publication/fuels-us-eu-cost-ekerosene-mar22 (accessed on 16 May 2025).
  89. Teter, J.; Cuenot, F.; McDonald, Z. Technology Roadmap: Hydrogen and Fuel Cells; International Energy Agency (IEA): Paris, France, 2015; Available online: https://www.iea.org/reports/technology-roadmap-hydrogen-and-fuel-cells (accessed on 16 May 2025).
  90. Kumar, H.; Kumar, H. To enhance the performance of ci engine with using of additives based hybrid bio fuel—A review. Macromol. Symp. 2024, 413, 2400067. [Google Scholar]
  91. Mujtaba, M.A.; Kalam, M.A.; Masjuki, H.H.; Gul, M.; Soudagar, M.E.M.; Ong, H.C.; Yusoff, M. Comparative study of nanoparticles and alcoholic fuel additives-biodiesel-diesel blend for performance and emission improvements. Fuel 2020, 279, 118434. [Google Scholar] [CrossRef]
  92. Zhang, Z.; Zhang, C.; Cai, P.; Jing, Z.; Wen, J.; Li, Y.; Zhang, J. The potential of coal-to-liquid as an alternative fuel for diesel engines: A review. J. Energy Inst. 2023, 109, 101306. [Google Scholar] [CrossRef]
  93. Gill, S.S.; Tsolakis, A.; Dearn, K.D.; Rodríguez-Fernández, J. Combustion characteristics and emissions of Fischer–Tropsch diesel fuels in IC engines. Prog. Energy Combust. Sci. 2011, 37, 503–523. [Google Scholar] [CrossRef]
  94. Benet, Á.; Villalba-Herreros, A.; d’Amore-Domenech, R.; Leo, T.J. Knowledge gaps in fuel cell-based maritime hybrid power plants and alternative fuels. J. Power Sources 2022, 548, 232066. [Google Scholar] [CrossRef]
  95. Neuling, U.; Bullerdiek, N. Classification of Power-to-Gas (PtG) and Power-to-Liquid (PtL) Processes. In Powerfuels; Springer: Cham, Switzerland, 2025; pp. 493–513. [Google Scholar]
  96. Jiao, Y.; Liu, R.; Zhang, Z.; Yang, C.; Zhou, G.; Dong, S.; Liu, W. Comparison of combustion and emission characteristics of a diesel engine fueled with diesel and methanol-Fischer-Tropsch diesel-biodiesel-diesel blends at various altitudes. Fuel 2019, 243, 52–59. [Google Scholar] [CrossRef]
  97. Ghadikolaei, M.A.; Cheung, C.S.; Yung, K.F. Study of combustion, performance and emissions of diesel engine fueled with diesel/biodiesel/alcohol blends having the same oxygen concentration. Energy 2018, 157, 258–269. [Google Scholar] [CrossRef]
  98. Han, S.; Kim, J.; Bae, C. Effect of air–fuel mixing quality on characteristics of conventional and low temperature diesel combustion. Appl. Energy 2014, 119, 454–466. [Google Scholar] [CrossRef]
  99. Inbanaathan, P.V.; Balasubramanian, D.; Wae-Hayee, M.; Veza, I.; Yukesh, N.; Kalam, M.A.; Varuvel, E.G. Comprehensive study on using hydrogen-gasoline-ethanol blends as flexible fuels in an existing variable speed SI engine. Int. J. Hydrogen Energy 2023, 48, 39531–39552. [Google Scholar] [CrossRef]
  100. Sharma, T.; Chauhan, P.S.; Patel, M.; Singh, A.; Kaur, M.; Chauhan, G.; Walia, A. Carbon negative biofuels: A step ahead of carbon neutrality. Biofuels 2025, 1–21. [Google Scholar] [CrossRef]
  101. Vijayashree, P.; Ganesan, V. Oxygenated fuel additive option for PM emission reduction from diesel engines—A review. In Engine Exhaust Particulates; Springer: Singapore, 2018; pp. 141–163. [Google Scholar] [CrossRef]
  102. Yang, R.; Yin, Z.; Yan, Y.; Ou, J.; Xie, T.; Liu, Z.; Liu, J. Investigating the Effect of Ammonia Addition on the Performance of a Heavy-Duty Natural Gas Spark Ignition Engine Operated at Stoichiometric Conditions. J. Energy Res. Technol. Part A. 2025, 1, 022306. [Google Scholar] [CrossRef]
Table 1. Overview of hybrid fuel components, production sources, blend ratios, and injection strategies used in CI engines.
Table 1. Overview of hybrid fuel components, production sources, blend ratios, and injection strategies used in CI engines.
Hybrid Fuel TypeComponentsProduction Source/MethodApprox. Blend RatioInjection/Combustion StrategyRefs.
Biofuel–hydrogenBiodiesel + H2Biodiesel: waste oils, fats, or crops; H2: electrolysis from renewables10–30% H2 in air (v/v), remainder biodieselDual-fuel (CI ignition of biodiesel, port-injected H2)[15,16]
Ammonia–hydrogenNH3 + H2NH3: green ammonia (renewable H2 + N2); H2: electrolysisNH3:H2 = 60:40 to 80:20 (v/v)Dual-fuel; port injection or premixed[15,16]
Ammonia–dieselNH3 + dieselNH3: green ammonia; diesel: fossil or bio-based10–40% NH3 (energy basis)Dual-fuel; diesel pilot ignition[16,20]
Ammonia–synthetic fuelNH3 + synthetic hydrocarbonsSynthetic fuels: CO2 capture + H2 via Fischer–TropschApprox. 30–70% NH3Single blended fuel; direct injection[16,21,22]
Biofuel–synthetic fuelBiodiesel + synthetic hydrocarbonsBiodiesel: waste/crops; synthetic fuels: renewable CO2 + H220–50% biodiesel commonSingle blend; direct injection[16,17,18]
Biodiesel–diesel (multi-component)Biodiesel + dieselBiodiesel: renewable; diesel: fossilB5 to B50 (5–50% biodiesel)Single blended injection[15,23]
Multi-component (e.g., NH3 + biodiesel + diesel)NH3 + biodiesel + dieselMix of renewable and fossil fuelsNH3 (10–30%) + Biodiesel (20–40%)Triple fuel: diesel pilot + port-injection[16,25]
Hydrogen–natural gas–biodieselH2 + CH4 + biodieselH2: electrolysis; CH4: biogas/natural gas; biodiesel: waste oilsOften 10–20% H2, 30–50% CH4Multi-injection; CI ignition[15,24]
Hydrogen–ammonia–dieselH2 + NH3 + dieselNH3 and H2: renewable; diesel: fossil or renewableApprox. 30% H2 + NH3; diesel for ignitionTriple fuel: multi-point injection[16,19,20]
Table 2. Efficiency performance of various hybrid fuels in CI engines.
Table 2. Efficiency performance of various hybrid fuels in CI engines.
StudyBlend TypeThermal EfficiencyPower OutputFuel ConsumptionNOx EmissionsCO2/CO EmissionsOther Pollutants
[62]Ammonia–hydrogenDecreased by 0.6 points (from H2 to 60% NH3)IMEP: 2.7 bar (H2), 3.1 bar (NH3–H2)Not mentionedNH3–H2: 1000–3500 ppm (vs. H2: <6 ppm)Not applicableN2O noted
[63]Ammonia–hydrogenConstant at 37% (MPRR > 9 bar/CAD)50% IMEP increase with NH3 vs. H2Not mentionedUp to 6000 ppm with NH3Not mentionedN2O < 1400 K
[64]Hydrogen, ammonia, biodieselMentioned (no values)Not mentionedBSFC mentioned (no values)Increased with biodieselCO2 decreasedHC decreased with biodiesel
[65]Hydrogen, natural gas, biodieselLower than dieselNot mentionedImproved BSECReducedNot mentionedHC and CO increased “up to several times”
[59]Ammonia–dieselIncreased with diesel substitutionNot mentionedNot mentionedIncreasedCO2 decreasedUnburned NH3: 14,800 ppm; N2O: 90 ppm
[53]Ammonia–hydrogen–dieselNot mentioned1.8% ↑ with 30% H20.3% ↓ with 30% H258.8% ↑ with 30% H2No CO2 changeHC and soot ↓
[66]Diesel, biodiesel, ammoniaDecrease in brake thermal efficiencyNot mentionedSlight ↑ in BSFC37% ↓ vs. dieselNot mentionedBSN ↓ by 53.5% vs. diesel
[67]Ammonia–diesel37.85% (ADDF) vs. 38.53% (diesel)Not mentionedNot mentioned58.8% ↓ (up to 40% NH3 fraction)CO ↓ by 20%N2O ↑; Unburned NH3: 4445 ppm
[68]Ammonia, biodiesel, diesel31.1% (diesel) → 33.3% (3 LPM NH3), 34.8% (6 LPM NH3)Not mentionedNot mentioned↓ with NH3 additionCO and CO2 ↓ with NH3HC emissions ↓ with NH3 and biodiesel
Table 3. Comparative summary of hybrid biofuel strategies for compression ignition engines.
Table 3. Comparative summary of hybrid biofuel strategies for compression ignition engines.
Hybrid ApproachPerformance BenefitsEmission ImpactChallenges/RequirementsReferences
Biofuel + hydrogenEnhances flame speed and thermal efficiencyReduces PM and CO2 emissionsHydrogen storage issues and low energy density[30,39,40,45]
Biofuel + ammoniaPotential for near-complete decarbonizationReduces NOx with dual-injection strategiesHigh ignition temperature; combustion stability; requires engine modifications[31,58,59]
Biofuel + synthetic fuelsImproved combustion efficiency, fuel atomization, and engine longevityLowers GHG emissions; cleaner combustionProduction cost; blend compatibility[32,81,82]
Biofuels + nano-additivesIncreased combustion stability, brake thermal efficiency, and reduced BSFCReduces emissions compared to conventional biodieselOptimization of additive concentration and dispersion[33]
Hydrogen (as a primary or secondary fuel)High flame speed; broad flammability improves efficiency and power outputNegligible CO2, reduced PM and CO; dual-fuel mode lowers sootHigh autoignition temp; pre-ignition risk; storage and safety systems needed[37,38,39,40,41,42,43,44,45]
Hydrogen + diesel (dual-fuel)Maintains diesel engine efficiency while lowering emissionsReduces soot; can control NOx with EGR strategiesInjection timing critical; requires knock suppression[42,43,44]
Hydrogen + biofuelsSynergistic effect: enhanced combustion and emission reductionsLowers PM and unburned hydrocarbonsNeeds precise fuel ratio control and chamber design optimization[38,39,40,45,50]
Ammonia (neat or blended)Compatible with existing infrastructure; potential for zero-carbon operationHigh NOx and N2O unless controlled; unburned NH3 possibleSlow flame speed, high ignition energy; needs pilot fuel, advanced ignition, or cracking[51,52,53,54,55,56,60,62,68,69,70,71,102]
Ammonia + hydrogenCombines ammonia’s zero-carbon profile with hydrogen’s high reactivityReduces NH3 slip; improves ignition; controls NOx with timing and ratiosAvoiding knocks and excessive NOx; requires combustion control strategies[60,62,71]
Synthetic fuels (e.g., FT, PtL, E-fuels)High cetane, low sulfur, reduced fuel consumption; compatible with CI enginesReduces PM, NOx, CO2, and SOxHigh production costs; needs CO2 capture and renewable energy sources[73,74,75,80,81,82,83,84,85,86,87]
Synthetic + biofuel blendsEnhanced BTE, ignition properties, and reduced engine wearLower CO2 footprint; mitigates NOx with proper formulationRequires testing for optimal blend ratios[90,91,92,93]
Overall hybrid blends (Bio + Syn + fossil)Improves engine metrics (BTE, BSFC, power output) depending on compositionCan increase or decrease NOx depending on oxygenates; overall CO2 and PM reduction possibleComplex fuel optimization: infrastructure and policy support needed[93,94,95,96,97,98,99,100,101]
Biofuel + hydrogenEnhances flame speed and thermal efficiencyReduces PM and CO2 emissionsHydrogen storage issues and low energy density[30,39,40,45]
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Khujamberdiev, R.; Cho, H.M. Hybrid Fuels for CI Engines with Biofuel Hydrogen Ammonia and Synthetic Fuel Blends. Energies 2025, 18, 2758. https://doi.org/10.3390/en18112758

AMA Style

Khujamberdiev R, Cho HM. Hybrid Fuels for CI Engines with Biofuel Hydrogen Ammonia and Synthetic Fuel Blends. Energies. 2025; 18(11):2758. https://doi.org/10.3390/en18112758

Chicago/Turabian Style

Khujamberdiev, Ramozon, and Haeng Muk Cho. 2025. "Hybrid Fuels for CI Engines with Biofuel Hydrogen Ammonia and Synthetic Fuel Blends" Energies 18, no. 11: 2758. https://doi.org/10.3390/en18112758

APA Style

Khujamberdiev, R., & Cho, H. M. (2025). Hybrid Fuels for CI Engines with Biofuel Hydrogen Ammonia and Synthetic Fuel Blends. Energies, 18(11), 2758. https://doi.org/10.3390/en18112758

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