Renewable Fuels for Internal Combustion Engines: 2nd Edition

1. Introduction

For many decades, internal combustion engines (ICEs) have served as the primary propulsion systems for mechanical vehicles and machines. Subsequent developments in ICE design have been directly linked to advances in materials, mechanical, thermodynamic, automation, and electronic engineering. However, the greatest challenge currently faced by ICEs is compliance with environmental protection requirements aiming at reducing the emissions of carbon dioxide (CO₂) and toxic compounds (NO_x, HC, PM). These constrains have prompted the search for new pathways to ICE development [1,2].

In recent years, there has been a growing interest in alternative fuels and hybrid power drives, which combine conventional engines with electric drives. Despite considerable advances in electromobility development, ICEs remain indispensable in many industrial sectors particularly heavy-duty transport, aviation, and agriculture where the energy density of fuel, durability, and distribution infrastructure play a key role [3,4].

According to the International Energy Agency [5], the global demand for energy will increase significantly in the coming years, including in the transport sector. The primary energy sources, including coal, crude oil, and natural gas, are expected to reach peak consumption by 2030, after which their share in the energy mix will decrease. However, it is unlikely that liquid fuels will be completely eliminated as the power source for ICEs by 2050. Therefore, current research focuses primarily on the search for alternative power sources and improving the energy efficiency of existing powertrains [6].

The following development pathways are currently being explored in ICE design:

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The search for new alternative fuels and conventional fuel additives, in particular those produced from various types of waste products [7,8];

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Computational fluid dynamics (CFD) simulations’ modeling flow and combustion processes in engines [9,10];

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The application of advanced combustion systems, such as Homogeneous Charge Compression Ignition (HCCI) and Reactivity Controlled Compression Ignition (RCCI), to achieve higher overall engine performance, while reducing the emissions of certain toxic compounds [11,12,13];

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The use of synthetic fuels that are carbon-free or are produced from CO₂ with the use of renewable energy (decarbonization) [14,15];

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Integration of ICEs with electric drives to develop plug-in and mild hybrids [16,17];

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New exhaust aftertreatment technologies, including the adaptation of SCR catalytic converters and GPD/DPF filters to new types of fuel [18,19].

This paper briefly reviews the articles published in the Special Issue of Energies, which is a continuation of the previous Special Issue [20]. The current Special Issue features twenty-one articles on various aspects of using renewable fuels in ICEs. The aim of this paper was not to discuss the research findings presented in these articles, but to overview the topics they address.

2. An Overview of Published Articles

Paredes-Rojas et al. (contribution 1) described the results of an experimental study investigating the applicability of B10 biodiesel in hybrid electric vehicles (HEVs) and proposing an Engine Management System (EMS) based on engine efficiency characteristics. According to the authors, the proposed solution optimizes fuel consumption and maintains an optimal battery state of charge (SOC). They argued that the B10 blend reduces toxic emissions without adversely affecting engine performance in HEVs.

Mohammed et al. (contribution 2) reported on the results of an experiment analyzing the combustion parameters, performance, and emission characteristics of a diesel engine fueled with blends of Croton macrostachyus (CMS, non-edible feedstock) seed oil biodiesel and diesel oil. Diesel oil, B20, B15, B20, and B25 blended fuels were tested at a constant speed under varying loads (0–80%). A higher biodiesel percentage increased peak cylinder pressure and the heat release rate, while decreasing ignition delay. Brake thermal efficiency (BTE) and brake-specific fuel consumption (BSCF) decreased, whereas NO_x emissions increased due to higher combustion temperature with an increase in biodiesel percentage. The authors concluded that CMS seed oil biodiesel is a promising alternative fuel that warrants further research.

Stanescu et al. (contribution 3) evaluated the performance and emission characteristics of a compression ignition engine fueled with biodiesel blends derived from used cooking oil (UCO) and sunflower oil (SF) at different proportions (0–50%). Biodiesel blends were tested under different load conditions and at two engine speeds. The experiment demonstrated that biodiesel blends containing up to 20% UCO and SF can be effectively used in diesel engines as they induce only a minor decrease in engine performance, while significantly reducing CO, HC, smoke, and CO₂ emissions. Nitric oxide emissions differed depending on fuel type and concentration. The study revealed that biodiesel blends containing more than 20% UCO and SF reduced smoke emissions but also led to a decline in engine performance and higher BSCF.

Jang et al. (contribution 4) analyzed the effects of decanol-blended diesel fuel on diesel engine performance and emission characteristics. Diesel fuel blended with 10%, 30%, and 50% decanol was tested in a single-cylinder engine at two speeds and under different loads. The authors found that increasing diesel blend percentage led to a decrease in engine performance, an increase in BSCF and brake-specific energy consumption (BSEC), a decrease in CO, HC, and NO_x emissions, and a significant decrease in smoke emissions. The authors concluded that low decanol blend ratios enhance engine performance, whereas higher blend ratios require a modified injection technology.

Kim et al. (contribution 5) evaluated the performance and emission characteristics of a diesel engine fueled with n-pentanol-diesel blends. The engine was operated at two speeds and different loads using three n-pentanol-diesel blends at volume ratios of 10%, 30%, and 50%. The study demonstrated that the analyzed diesel blends improved selected engine performance parameters.

Jamrozik and Tutak (contribution 6) reviewed the literature on the impact of alcohol content in diesel blends. They evaluated the influence of alcohol concentration on the properties of the resulting biofuel mixture, engine performance, emission characteristics, and the fuel injection system. The authors concluded that alcohols added to diesel oil contribute to reducing CO and particulate matter (PM) emissions in diesel engines.

Kozak et al. (contribution 7) analyzed the emission characteristics of a Euro 5 direct injection spark-ignition engine fueled with conventional gasoline (E0) and a gasoline-ethanol blend with 30% v/v ethanol content (E30). Gaseous emissions (CO, HC, NO, CO₂), particulate number (PN), and fuel consumption were determined for each fuel type. In an engine equipped with a three-way catalyst, E30 fuel did not affect emission characteristics (CO, HC, NO) but reduced CO₂ emissions due to its more favorable hydrogen-to-carbon ratio. E30 significantly reduced PM emissions by 40% at idle speed and by 80–90% under high load. This gasoline–ethanol blend does not require engine modification and can be safely applied in existing vehicles with direct-injection engines. The authors emphasized that ethanol can be produced from various types of biomass using existing technologies, and gasoline–ethanol blends help reduce the carbon footprint and support the achievement of climate neutrality goals.

Muhssen et al. (contribution 8) comprehensively reviewed recent research on tri-fuel diesel engines utilizing hydrogen-enriched natural gas (HNG). They observed that co-combustion of natural gas (NG) and hydrogen with diesel fuel improves the combustion of lean mixtures typical of dual-fuel engines (NG + diesel oil), which reduces toxic emissions.

Wang-Alho et al. (contribution 9) investigated the effects of alternative fuels and their combinations on metal corrosion and the influence of corrosion products on fuel properties. They analyzed the corrosion of aluminum, carbon steel, stainless steel, and the MoC210M/25CrMo4+SH alloy under exposure to methanol and hydrotreated vegetable oil (HVO) blends. The authors found that methanol increases corrosion risk and can contribute to the dissolution of aluminum. In turn, HVO had no adverse effects on the tested metals or their alloys.

Dimitriadis et al. (contribution 10) evaluated synthetic fuels as potential feedstocks for road transportation, aviation, and the maritime sector. The study demonstrated that feedstocks simulated through hydrotreatment had similar composition and properties to existing fuels. A storage stability analysis revealed that hydrotreated fuels can be stored for more than six months without noticeable changes in their properties.

Dimitriadis et al. (contribution 11) examined the production of e-fuels via hydrocracking of wax from the Fischer–Tropsch (FT-wax) synthesis. Hydrogen for the hydrocracking process was obtained by water electrolysis powered by solar energy. The liquid product was fractionated to obtain fuels that met the specifications of petroleum-based gasoline, diesel, and heavy fuel oil. The study also demonstrated that the obtained fuels can be stored without any change in their properties.

Tutak and Jamrozik (contribution 12) reviewed the literature on the use of ammonia for powering ICEs. They found that at the current level of technological development, ammonia can be used as fuel only in dual-fuel engines. The authors concluded that ammonia can account for up to 60% of the energy input in diesel engines to achieve acceptable NO_x emission levels, thus contributing to a significant reduction in greenhouse gas emissions.

Dybiński et al. (contribution 13) comprehensively reviewed the literature on the market of synthetic fuels, also referred to as e-fuels. They observed that although the e-fuel market is currently in the early stages of development, the market share of e-fuels could reach 10% within the next five years and potentially 30% over the next two decades. E-fuels could play a critical role in the decarbonization of the transportation sector, but further research, collaboration, and policy alignment are needed to achieve that goal.

In a review article, Chavando et al. (contribution 14) analyzed the advantages and challenges associated with the use of ammonia (NH₃) as fuel in the marine sector. The benefits of ammonia are discussed in the context of decarbonization and sustainable development of the shipping industry. The authors emphasized that green ammonia can be produced using renewable energy sources such as wind and solar power.

Chavando et al. (contribution 15) proposed a simulation model of a continuous pyrolysis reactor for a heat self-sufficient process. The model was developed on the assumption that pyrolysis should generate sufficient heat to sustain continuous operation and produce fuel. The simulation revealed that when heat streams and heat recovery are appropriately configured, the energy balance can approximate self-sufficiency. However, according to the authors, the above outcome is possible only under specific circumstances and that the proposed solution may not always be economically viable.

Kakoee et al. (contribution 16) analyzed the effect of start of ignition (SOI) on fuel distribution in the combustion chamber, engine performance, and pollutant emissions in the Reactivity Controlled Compression Ignition (RCCI) technology for marine application. The analysis relied on CFD, and the developed model was validated based on the experimental results. The study revealed that SOI is a key parameter in RCCI engines, affecting both engine performance and toxic emissions.

Mattarelli et al. (contribution 17) presented the virtual development of an opposed-piston (OP) hydrogen engine designed to deliver a power output of 25 kW and predict its performance through simulations. The main design criteria were high engine performance and low NO_x emissions. The simulation revealed that the designed engine configuration met the performance targets and achieved NO_x emissions below 20 ppm. According to the authors, the proposed OP engine would be lighter and less expensive to produce than a conventional four-stroke engine.

Szwajca et al. (contribution 18) analyzed the parameters of an electric discharge arc used to ignite lean fuel–air mixtures. They conducted an experiment investigating the flow of electrical energy generated by the ignition system and the parameters of an electric discharge arc. The results of the study indicate that spark discharge energy is strongly influenced by ambient conditions (charge pressure) and ignition parameters (charging time, voltage), which are particularly important during the ignition of lean mixtures or gaseous fuels. Optimization of charging time and ignition coil design could increase ignition energy, leading to improved combustion (particularly for lean mixtures) and potentially lower emissions.

Ricci and Mariani (contribution 19) examined the detection of flame front evolution in a spark-ignition (SI) engine using an autoencoder, an innovative neural network. They concluded that the autoencoder outperforms established neural networks, particularly in its ability to capture flame development. The proposed neural network improves monitoring and enhances combustion, thereby reducing emissions and improving the effectiveness of SI engines.

Weerakoon and Assadi (contribution 20) analyzed the potential of micro gas turbines (MGTs) in low-emission energy systems and their integration with distributed energy generation systems. The authors concluded that to fully harness the potential of MGTs, further research is needed to reduce their cost and improve their competitiveness relative to ICEs and renewable sources such as solar and wind power.

The last contribution by Obidzinski et al. (contribution 21) diverges from the topics addressed in the preceding articles because it examines the effects of adding potato pulp (a waste product from the potato industry) to pine sawdust on the physical properties and energy efficiency of fuel pellets. The results of the study indicate that incorporating potato pulp into biomass could offer a promising solution for waste management and renewable fuel production.

3. Conclusions

In summary, many of the articles published in the Special Issue of Energies on the use of renewable fuels in ICEs explore various additives to conventional fuels with the aim of reducing the share of fossil fuels in the overall energy mix. However, compared with the first Special Issue [20] published in 2020–2021, the present Special Issue introduces new solutions that reflect current trends in ICE development.

In this Special Issue, several articles discuss synthetic fuels, or e-fuels, that align with the decarbonization concept. The above points to the growing interest in ammonia as an alternative fuel characterized by high energy density and ease of storage.

Some of the published articles suggest that numerous simulation studies are being conducted to optimize new fuel combustion technologies, with the aim of enhancing engine performance and reducing toxic compound emissions into the atmosphere. Intensive research on hybrid drives contributes to the optimization of ICEs. Recent research on novel engine and gas turbine designs can also enhance performance in specific applications.

An overview of the Special Issue and other articles [6] suggests that despite the emergence of alternative drives, including the electric drive, ICEs will remain the primary propulsion systems for mechanical vehicles and machines in the coming decades.