Comprehensive Review of Hydrogen and Tyre Pyrolysis Oil as Sustainable Fuels for HCCI Engines

Abstract

This review article provides an overview of the use of hydrogen and tyre pyrolysis oil as fuels for homogeneous charge compression ignition (HCCI) engines. It discusses their properties, the ways they are produced and their sustainability, which is of particular importance in the present moment. Both fuels have certain advantages but also throw up many challenges, which complicate their application in HCCI engines. The paper scrutinises engine performance with hydrogen and tyre pyrolysis oil, respectively, and compares the fuels’ emissions, a crucial focus from an environmental perspective. It also surveys related technologies that have recently emerged, their effects and environmental impacts, and the rules and regulations that are starting to become established in these areas. Furthermore, it provides a comparative discussion of various engine performance data in terms of combustion behaviour, emission levels, fuel economy and potential costs or savings in real terms. The analysis reveals significant research gaps, and recommendations are provided as to areas for future study. The paper argues that hydrogen and tyre pyrolysis oil might sometimes be used together or in complementary ways to benefit HCCI engine performance. The importance of life-cycle assessment is noted, acknowledging also the requirements of the circular economy. The major findings are summarised with some comments on future perspectives for the use of sustainable fuels in HCCI engines. This review article provides a helpful reference for researchers working in this area and for policymakers concerned with establishing relevant legal frameworks, as well as for companies in the sustainable transport sector.

1. Introduction

All around the world, a strong movement towards greater use of sustainable energy sources is currently underway, and this is pushing various actors to start looking seriously at alternative fuels for use in internal combustion engines. Among these options, hydrogen and tyre pyrolysis oil (TPO) show particular promise, especially for homogeneous charge compression ignition (HCCI) engines. The purpose of this review article is to introduce these two fuels and explore their fitness for HCCI systems, discussing their properties, manufacture and environmental impacts. HCCI engines provide a good combination of high efficiency with low emissions, which makes them interesting candidates as alternative fuels [1]. Hydrogen is a clean fuel because when it burns, no carbon is emitted; it holds much promise for eliminating greenhouse gases from the transport sector [2,3]. Meanwhile, TPO, which is made from waste tyres, fits into the circular economy by both providing a fuel source and facilitating waste management.

In 2024, the global TPO market saw fast growth. This was partly because of increased concerns about the environment but also because of increasing demand for better waste management solutions. The market was estimated at about USD 363.2 million in 2024, and it is expected to grow to USD 611.9 million by 2034 at a compounded annual growth rate (CAGR) of over 5.4% [4], as more and more industrial actors, such as power plants, cement factories and steel makers, adopt TPO as a fuel source. The market is seeing solid growth especially in the Asia–Pacific region, where rapid industrial growth has been accompanied by supportive government policies.

TPO manufacture involves a process of pyrolysis, whereby end-of-life tyres are heated at high temperatures without oxygen, resulting in oil production. Usually, this process yields roughly 30–65% oil, 25–45% char and 5–20% gas output [5]. Given that globally, more than 26 million tonnes of new tyres are produced every year, the supply of scrap tyres for TPO production is stable. Recent pyrolysis technologies have also made the process more efficient and environmentally safer, making TPO production more viable and attractive for both energy recovery and the mitigation of waste problems. As circular economy models remain prominent globally, TPO production seems set to increase in importance for global sustainable development strategies.

In the present industrial situation in India, TPO is attracting increasing attention as a potential alternative fuel. One reason for this is that fuel prices keep rising, but the country is also generating huge amounts of waste tyres every year—about 275,000 to 300,000 tonnes in 2024. This positions India among the biggest waste-tyre producers in the world. As mentioned, the pyrolysis process turns these waste tyres into products such as oil, which consists of about 40–45% of the output by weight. The oil is used mainly in industrial heating, brick kilns and some modified diesel engines, although regulations prevent its full-scale, mainstream use as fuel.

Table 1. Tyre pyrolysis oil market report attributes [4].

Report Attribute	Details
Base year	2024
Tyre pyrolysis oil market size in 2024	USD 363.2 million
Forecast period 2025–2034 CAGR	5.4%
2034 value projection	USD 611.9 million
Historical data period	2021–2024
Segments covered	Raw material, process, end use and region
Growth drivers	Environmental concerns and waste disposal issues Rising fuel prices and economic benefits Versatility and adaptability in various applications
Pitfalls/challenges	Technological limitations and production efficiency

shows, the TPO sector is mostly unorganised, with more than 250 small-to-medium pyrolysis plants operational in different parts of the country. These plants help recycle waste tyres but often run without strict environmental monitoring. However, thanks to better awareness and improved technology, the sector is slowly becoming more organised. The Central Pollution Control Board, for example, has recently implemented tighter environmental regulation for these plants under the Hazardous Waste Management Rules, requiring them to obtain approvals. With the Indian government paying more attention to circular economy and green energy models, TPO seems likely to begin playing a larger role if the industry can comply with the new environmental standards and adopt product standardisation.

This review article provides an overview of the behaviour of hydrogen and TPO during combustion in HCCI engines, their respective performance levels and emissions. It introduces the various issues connected with making the use of these fuels work well in practice, such as storage, transport, distribution and the need to adapt engine design to optimise performance. The study also surveys recent technological developments that have increased the feasibility of the use of these fuels in HCCI engines. It discusses how each fuel burns and how it is manufactured, as well as their respective life-cycle effects on the environment in consideration of broader sustainable development targets. Thus, the article addresses both upstream and downstream consequences of adopting these fuels. It also examines rules and policies currently in place around the use of alternative fuels and assesses their potential influence on the expansion of hydrogen or TPO usage in the automobile sector [6].

Bringing all these elements together, the article offers insights that will be useful for researchers studying these fuels, for policymakers engaged in shaping fuel policies and for industrial actors currently deciding whether to invest in or adopt such technologies. It highlights important research gaps and makes recommendations for future areas of investigation. Finally, the article suggests that combining hydrogen and TPO might provide a synergy effect to promote the development of sustainable transportation options. Figure 1 shows a comparison between the HCCI engine, the Spark Ignition (SI) and the compression ignition engine.

1.1. Background of Sustainable Fuels and Homogeneous Charge Compression Ignition Engines

The growing movement worldwide towards more sustainable energy pathways has brought a stronger focus on the use of alternative fuels for internal combustion engines. Among many options now being considered, hydrogen and TPO have emerged as potential candidates for use in HCCI engines. HCCI engines are known mainly for having high efficiency levels and low emission outputs, desirable qualities for alternative fuels [8]. Hydrogen, one of the cleanest fuels due to its zero carbon emissions, has strong potential to help decrease greenhouse gas levels produced through the transportation sector [9]. Several methods exist for the production of hydrogen, one such being water electrolysis using renewable energy, which can make the fuel even more sustainable in some cases. However, several issues give rise to complications, for example, storage, distribution or requirements for special infrastructure. These can create significant obstacles to the widespread use of hydrogen fuel. TPO, on the other hand, is obtained from waste tyres and is compatible with the circular economy, such that waste management problems and energy demands can be addressed together. TPO is produced through the pyrolysis process, where waste tyres undergo thermal decomposition under oxygen-free conditions, resulting in the production of oil, gas and char. This method not only creates usable fuel but also helps mitigate the growing issue of tyre waste, which is becoming problematic in many regions [10].

When these respective fuels are used for HCCI engines, they show different combustion behaviours. Hydrogen, mainly because of its wide flammability window and high flame speed, burns very efficiently, which usually results in better engine output and, to some extent, reduces emissions. TPO’s more complex chemical composition often causes variation in combustion inside the engine, so careful calibration is required to balance performance and emissions levels [11,12]. Although both fuels hold promise, several practical obstacles to their direct implementation in real-world HCCI systems remain. For hydrogen, special designs are required not only for storage but also for delivery into the engine cylinder. This is mostly due to its unique combustion properties and physical features, including low molecular weight and high diffusivity. Engine modifications are also necessary to control the fuel’s fast ignition and specific combustion characteristics. TPO, on the other hand, usually demands extra processing steps. Cleaning operations may be required to remove impurities, and stabilisation processes are often applied to ensure a relatively uniform chemical makeup before burning. Without these treatments, stable combustion with TPO becomes much harder to maintain. Engine subsystems such as fuel injection hardware and combustion control modules may also need to be adjusted in order to handle TPO’s behaviour more precisely during combustion. In recent years, technological improvements have rendered both fuels as more practical options for use in HCCI engines. Potentials for hydrogen storage, for example, have been improved by the development of high-pressure storage cylinders and metal hydride storage systems; together, these innovations resolve some of the problems of on-board storage that have blocked widespread hydrogen application. For TPO, better pyrolysis processing and oil upgrading techniques have helped improve the fuel’s consistency, making it more stable and better suited for advanced combustion systems such as HCCI engines.

However, questions remain over environmental impacts. Although hydrogen produces only water vapour when it burns, hydrogen manufacture can still have diverse environmental impacts, depending on the energy source. TPO helps control waste-tyre build-up, but the pyrolysis process can release emissions and needs proper controls. Therefore, full life-cycle assessment (LCA) is essential to understand more fully the positive and negative environmental aspects [13,14]. Regulation also plays a critical role in determining how quickly these fuels might find a place in the market. Policies that support low-carbon energy and ideas of circular economy may offer incentives that encourage the production and adoption of hydrogen and TPO for HCCI engines [15,16]. Safety concerns must also be addressed, especially for hydrogen due to its sensitivity in storage and handling. Nevertheless, hydrogen and TPO both remain promising sustainable fuel options for HCCI applications. The potential of these fuels to help cut emissions and deal with waste will continue to make them interesting for future transportation solutions. However, more research is required, alongside new technical solutions, in order to deal with the remaining challenges and fine-tune the fuels’ performance in HCCI engines. As the automotive sector moves slowly but surely towards cleaner and more sustainable operations, these alternative fuels are likely to remain the focus of investigation and development for many years to come [17,18].

1.2. Importance of Hydrogen and Tyre Pyrolysis Oil as Alternative Fuels

Both hydrogen and TPO have started to stand out as potential alternatives in the search for sustainable energy sources. Hydrogen is a clean-burning fuel that produces nothing but water vapour when used in combustion engines and fuel cells, which makes its usage appealing as a means of decreasing greenhouse gas emissions. Its high energy density and flexibility allow it to be used in many different areas, including transportation, heavy industry and even power generation in certain systems. However, despite these promising potentials, many barriers prevent hydrogen’s large-scale use. These include high production costs, challenges in storage and a lack of proper distribution networks [19].

TPO, manufactured by thermally decomposing waste tyres under controlled conditions, provides what might be called a double advantage, addressing issues of waste management and energy generation simultaneously. This oil can substitute for conventional fossil fuels, for example, in industrial boilers, furnaces and some specially adapted diesel engines. TPO manufacture helps lower the environmental load caused by discarded tyres while simultaneously creating useful energy products. Its use contributes directly to the circular economy because it converts waste materials into valuable fuel, thereby decreasing reliance on conventional fossil fuel sources. However, there is a need for further research to make the production process more efficient, enhance fuel quality and ensure that emission standards are being met for broader and safer application across industries [20].

2. Hydrogen as a Sustainable Fuel for Homogeneous Charge Compression Ignition Engines

Hydrogen has emerged as one of the most promising sustainable fuels for HCCI engines, mainly because of its clean-burning nature and its potential to promote zero-emission transport systems. HCCI engines combine certain operating principles from gasoline and diesel engines and stand to gain from the characteristics of hydrogen combustion. Because of hydrogen’s high flame speed, together with its wide flammability limits, the combustion process remains efficient even under a broad range of engine working conditions [21]. Furthermore, hydrogen exhibits very low ignition energy and a high auto-ignition temperature; this makes it a good fit for HCCI set-ups, allowing a more flexible combustion process and potentially less complicated engine management systems [9]. When hydrogen is burned in HCCI engines, one major benefit is the elimination of carbon emissions, as the main by-product is simply water vapour. However, this does not mean that the road ahead is clear, as issues remain that complicate hydrogen adoption. Problems such as how to ensure safe storage, how to build the required distribution infrastructure and how to deal with potential nitrogen oxide (NOx) emissions, particularly under higher load conditions, still demand attention. At present, research activities are ongoing to optimise the design of hydrogen HCCI engines, improve the fuel delivery mechanisms and develop advanced control strategies towards realising the full potential of hydrogen as a sustainable fuel option [22].

2.1. Properties of Hydrogen Relevant to Homogeneous Charge Compression Ignition Combustion

Hydrogen has certain unique properties that are potentially very useful for HCCI combustion systems. One such is hydrogen’s very high diffusivity, which enables it to mix quickly with air and form an evenly distributed fuel–air mixture inside the combustion chamber. Such uniformity in the mixture is important for HCCI functioning, facilitating near-simultaneous ignition throughout the chamber space. Moreover, hydrogen has very wide flammability limits, ranging from 4% to 75% by volume when mixed with air, which allows the engine to run even under very lean conditions. Ultra-lean operation, in turn, may help to reduce NOx emissions and may also promote thermal efficiency. The high auto-ignition temperature of hydrogen (585 °C) allows for higher compression ratios, further enhancing thermal efficiency. Additionally, hydrogen’s high laminar flame speed contributes to faster combustion, which can help mitigate the challenges associated with controlling ignition timing in HCCI engines [23]. This combustion efficiency, which varies according to the mixture components, is illustrated in Figure 2.

However, hydrogen’s properties also present challenges for HCCI combustion. Its low energy density by volume necessitates larger fuel storage systems or more frequent refuelling. Its high burning velocity can lead to rapid pressure rise, potentially causing excessive engine noise and structural stress. Furthermore, hydrogen’s low minimum ignition energy increases the risk of pre-ignition and backfiring, requiring careful control of the mixture temperature and pressure during the compression stroke. These characteristics demand sophisticated engine management systems to fully exploit hydrogen’s potential in HCCI applications while maintaining stable and efficient operations across various load conditions [24].

2.2. Production Methods and Sustainability Aspects

Hydrogen production for HCCI engines involves steam methane reforming, electrolysis and biomass gasification, each process revealing distinct sustainability implications. Steam methane reforming, while economical and efficient, relies on fossil fuels and emits significant amounts of carbon dioxide (CO₂). Electrolysis, powered by renewable energy, offers a clean, potentially carbon-neutral method but faces challenges in the form of high capital costs and infrastructure limitations. Biomass gasification provides a potentially carbon-neutral approach by converting organic matter into hydrogen-rich syngas, but problems include feedstock availability, process efficiency and land use. The sustainability of hydrogen production depends on the respective energy sources and efficiency levels [25]. Figure 3 shows the various hydrogen production methods.

Efforts towards developing green hydrogen methods have yielded advancements in electrolysis technologies and photocatalytic water splitting. Implementing Carbon Capture and Storage (CCS) technologies in fossil fuel-based production can mitigate environmental impacts but faces obstacles in terms of cost-effectiveness and storage stability.

Efficient hydrogen transportation and storage are crucial for HCCI engines’ sustainability, requiring advanced compression techniques, liquid hydrogen systems or chemical carriers. As technologies advance, a combination of methods will likely be required to meet the growing demand for sustainable hydrogen fuel, responding to local resources, infrastructure and energy policies. Continued investment in research and development and supportive policy frameworks are necessary to realise hydrogen’s potential as a clean fuel for HCCI engines [27,28,29].

Overconsumption by humans has terribly degraded the condition of the natural environment and contributed to climate change. Green hydrogen, especially as manufactured through water electrolysis powered by renewable energy, is an interesting fossil fuel substitute [30].

2.3. Advantages and Challenges of Hydrogen in Homogeneous Charge Compression Ignition Engines

Because hydrogen has a wide flammability range, the engine can potentially run under lean conditions, which can help boost fuel efficiency. Additionally, its high auto-ignition temperature combined with its fast flame speed allows for better control during combustion: ignition timing can be managed with more accuracy, sometimes even allowing for the use of higher compression ratios. These features make hydrogen an attractive option for fuelling HCCI engines, offering opportunities for cleaner and more efficient operations when compared with regular fossil fuels [31]. Figure 4 explains the various challenges associated with HCCI engines.

Despite hydrogen’s many advantages, serious challenges remain in relation to its use in HCCI engines. One of the larger problems is the difficulty of controlling exactly when combustion occurs, as hydrogen is very reactive. This sometimes causes premature ignition or knocking, which can harm engine components. Another issue arises from hydrogen’s low density, which demands either larger tanks or high-pressure systems to ensure enough fuel can be stored on board the vehicle. Moreover, the small size of hydrogen molecules increases the risk of leakage, so specialised seals and storage designs are necessary to prevent loss and ensure safety. Not only this, but hydrogen production, distribution and storage all involve infrastructure and cost issues, which continue to provide major barriers to more widespread adoption. Solving these problems will require further research and technological advancement, especially in combustion control methods and the areas of hydrogen storage and infrastructure [33].

2.4. Performance Characteristics of Hydrogen-Fuelled Homogeneous Charge Compression Ignition Engines

Hydrogen has several advantages for use in HCCI engines. Its high diffusivity means that the fuel–air mixture inside the combustion chamber is uniformly distributed, which leads to more efficient combustion and reduced emissions, especially of particulate matter (PM) and NOx. Hydrogen’s wide flammability range enables operation under lean conditions, further improving fuel efficiency. Additionally, hydrogen’s high auto-ignition temperature and fast flame speed contribute to better control over the combustion process, allowing for more precise timing and potentially higher compression ratios [34]. The influence of syngas composition on combustion pressure at fixed ø and T_ivc is shown in Figure 5a.

An increase in hydrogen fraction in the syngas results in increased combustion pressure. Figure 5a also shows that an earlier start of ignition is achieved with a higher hydrogen content in the synthesis gas. The sensitivity analysis suggests that the hydrogen radical has a significant influence on the combustion process. Figure 5b shows the variation in gas emissions (CO₂, CO, NOx) compared to the H₂/CO mixture.

The reduction in CO₂, CO and NOx pollutants are the functions of the increase in the hydrogen content in the fuel (H₂/CO ratio).

However, the use of hydrogen in HCCI engines also presents significant challenges. One major issue is the difficulty of controlling the combustion timing due to hydrogen’s high reactivity. This can lead to premature ignition or knocking, potentially damaging the engine. The low density of hydrogen requires larger storage tanks or high-pressure systems, posing challenges for onboard storage in vehicles. Additionally, hydrogen’s small molecule size increases the risk of fuel leakage, necessitating specialised sealing and storage solutions [33]. Figure 6 explains the operating characteristics of HCCI engines.

Overcoming these hurdles requires continued research and development in areas such as combustion control strategies, fuel storage technologies and infrastructure. Advancements in these areas can potentially unlock the full potential of hydrogen-fuelled HCCI engines, leading to cleaner and more efficient transportation solutions. However, the economic viability and widespread adoption of this technology will depend on the extent to which the technical challenges and broader infrastructural issues related to hydrogen production and distribution can be addressed [36].

2.5. Emissions Profile of Hydrogen Homogeneous Charge Compression Ignition Engines

Hydrogen-fuelled HCCI engines offer a promising pathway for reducing emissions in the transportation sector. The emissions profile of these engines is characterised by significantly lower levels of certain pollutants compared with conventional internal combustion engines. Notably, hydrogen HCCI engines produce virtually zero CO₂ emissions during operation, as the combustion of hydrogen with oxygen yields only water vapour as a by-product. Because of this property, hydrogen HCCI engines are an attractive choice to help cut greenhouse gas emissions [12].

With regard to other pollutants, hydrogen HCCI engines mostly exhibit very low amounts of carbon monoxide (CO) and unburned hydrocarbons: these are usually associated with the incomplete burning of carbon-based fuels, which does not apply with hydrogen. However, the formation of NOx remains one issue that needs attention. Although the lean-burning nature of HCCI engines usually works to reduce NOx levels compared with traditional engines, some NOx may still be formed on account of the very high combustion temperatures that hydrogen can reach. The amounts produced depend on several factors, such as the leanness or richness of the air–fuel mixture, the temperatures reached during combustion and the conditions under which the engine operates. Researchers are currently working on ways to reduce NOx further in hydrogen HCCI engines, for example, by applying advanced combustion control methods or exhaust gas recirculation (EGR) techniques [37,38].

Despite hydrogen HCCI engines’ already very low overall emission profile, they can still produce small amounts of other pollutants from engine oil residues or impurities in the hydrogen fuel itself. While these emission levels are normally very low when compared with conventional engines, they may still merit further investigation, especially given strict ultra-low emission regulations. In addition, small quantities of unburned hydrogen may sometimes escape during combustion. Although unburned hydrogen is not directly harmful to the environment, it may have an indirect environmental impact if the source is not fully renewable. Current research is attempting to improve the combustion process and engine design, aiming to achieve further decreases in all types of emissions and additional improvements in the environmental performance of hydrogen HCCI engines [39].

3. Tyre Pyrolysis Oil as a Sustainable Fuel for Homogeneous Charge Compression Ignition Engines

TPO has begun to show potential as an alternative fuel option for HCCI engines, offering a sustainable way to handle both waste management and energy demands. TPO is produced through the thermal decomposition of waste tyres in the absence of oxygen, which breaks down the tyres into a liquid fuel with properties similar to conventional petroleum-based fuels. This process, called pyrolysis, works to recycle end-of-life tyres and yields valuable energy resources, contributing to the circular economy while reducing the environmental burden caused by tyre dumping and disposal [40].

TPO brings several benefits when used in HCCI engines, especially from a sustainability perspective. These engines are well known for delivering high thermal efficiency while keeping emissions such as NOx and PM relatively low. Using TPO as fuel, they retain these efficiency benefits while running on a renewable fuel source. The chemical structure of TPO, which comprises aliphatic and aromatic hydrocarbons, supports atomisation and vaporisation, which are important for creating a homogeneous mixture inside the HCCI combustion chamber. Because of this compatibility, TPO can typically be used with very few modifications in existing HCCI engine designs, which facilitates its adoption as a sustainable fuel alternative [41]. The most important properties of TPO are shown in

Table 2. Important properties of TPO fuel [41].

Properties	Values
Density at 15 °C (kg/m3)	900 to 935
Kinematic viscosity at 40 °C (cSt)	2.81 to 6.3
Calorific value (MJ/kg)	42 to 43.27
Flashpoint (°C)	20 to 43
H/C ratio	1.28 to 1.6
Moisture (wt.%)	4.6
Sulphur (wt.%)	0.6 to 1.45
Carbon residue (wt.%)	2.14 to 2.2
Aromatic content (wt.%)	64

.

Although TPO has clear potential, challenges remain. One main issue is that TPO composition is not always constant; it varies depending on the nature of the pyrolysis process and the kind of tyres used as feedstock. This variability can result in shifts in engine performance and emission levels. Researchers are currently working on standardising the process of TPO production and developing better fuel treatment techniques to help stabilise fuel quality. Another challenge comes from TPO’s physical characteristics: it often shows higher viscosity and a lower cetane number compared with conventional diesel fuels. Such differences sometimes require engine modification, for example, adjustment of the injection pressure or the injection timing in order to achieve efficient combustion while maintaining optimum performance [42].

Even with all these challenges, the promise of TPO as a sustainable fuel for HCCI engines is substantial. It addresses the issue of waste-tyre management while simultaneously promoting decreased reliance on fossil fuels. As more studies are conducted, both TPO production methods and engine control strategies will likely improve, making this fuel option even more practical for real-world use. TPO will then be able to play an important role in pushing transportation towards greater sustainability, contributing to global efforts to reduce carbon emissions and supporting circular economy approaches within the auto sector [43].

3.1. Composition and Properties of Tyre Pyrolysis Oil

TPO is a complex mixture of hydrocarbons obtained from the thermal decomposition of waste tyres in the absence of oxygen. The composition and properties of TPO can vary depending on the conditions of the pyrolysis process, tyre composition and post-treatment methods. Generally, TPO consists of aliphatic and aromatic hydrocarbons, with carbon chain lengths ranging from C5 to C20. The major components of TPO include (1) aliphatic hydrocarbons: straight-chain and branched alkanes and alkenes; (2) aromatic hydrocarbons: single-ring (e.g., benzene, toluene and xylenes) and polycyclic aromatic compounds; and (3) heteroatom-containing compounds: sulphur-, nitrogen- and oxygen-containing molecules.

TPO typically has a dark brown or black colour and a strong odour due to the presence of sulphur compounds. Its density ranges from 0.9 to 0.98 g/cm³, which is slightly lower than that of water [44]. The viscosity of TPO is generally higher than conventional diesel fuel, ranging from 3 to 8 cSt at 40 °C. The heating value of TPO is comparable to that of conventional petroleum-based fuels, typically ranging from 40 to 44 MJ/kg [45]. This high energy content makes it attractive as an alternative fuel source. However, TPO often has a lower cetane number compared with diesel fuel, which can affect its ignition quality in compression ignition engines [46].

In addition, TPO carries trace amounts of heavy metals, such as zinc, lead and chromium, which mostly come from the metallic parts and chemical additives used during the tyre manufacturing process. If these metals are not properly managed or removed during fuel processing, they may end up contributing to engine wear problems and may also influence emission levels. In terms of chemical and physical characteristics, TPO emerges as a potential substitute for regular fuels in different applications, including internal combustion engines. For direct application to engines, however, certain adaptations need to be made, either by modifying the engine system or by processing the fuel beforehand to better deal with the challenges that come from TPO’s specific composition and variable properties [47,48,49].

3.2. Production Process and Environmental Implications

Production of TPO usually takes place through the thermal decomposition of waste tyres in an oxygen-free environment. The process begins with the shredding of the waste tyres into smaller pieces, which helps increase the surface area and allows for better heat transfer during the later stages [50]. After shredding, the tyre chips are placed in a pyrolysis reactor, where the temperatures are raised to between 400 and 700 °C, depending on the set-up. As the temperature rises, the organic compounds in the tyres break down into smaller molecules, forming a mixture of gases, liquids and solid residues. The gases are condensed to form TPO, while the remaining non-condensable gases can be used as fuel for the pyrolysis process itself. The solid residue, known as char or carbon black, can be further processed for various applications. Figure 7 illustrates the process [51].

Pyrolysis provides a workable solution for the growing problem of waste-tyre disposal—tyres often otherwise end up being dumped in landfills or incinerated. By converting old tyres into usable products, the pyrolysis approach thus reduces environmental pressures while also serving circular economy models. Furthermore, the use of TPO as a fuel can help reduce reliance on fossil fuels, supporting energy security and reducing greenhouse gas emissions linked with the extraction and refining of conventional petroleum fuels [52].

On the other hand, environmental concerns remain. If the system is not controlled correctly during pyrolysis, it can release volatile organic compounds and PM into the atmosphere, directly contributing to pollution levels. Another serious problem emerges from TPO’s commonly rather high sulphur content: when burned, TPO may produce SO₂ emissions, causing air pollution and acid rain formation. The heavy metals present in TPO, such as zinc, lead and chromium, may also pose environmental threats if not managed carefully, since these can sometimes contaminate the soil or enter water systems. Manufacture and burning may also release polycyclic aromatic hydrocarbons (PAHs), which are well-known carcinogens and very harmful for both humans and ecosystems. To manage these challenges, stricter environmental regulations are required, combined with better technological solutions. Devices such as scrubbers and filters can be installed to help trap and lower emissions released during the pyrolysis process. Desulphurisation techniques may reduce the sulphur content of TPO, allowing clean and safe burning. Advanced catalytic upgrading processes can improve the quality of TPO fuel while also reducing its total environmental burden. Complete LCAs should be conducted so that the full picture of TPO’s overall environmental impacts can be compared against conventional fossil fuel production routes, as well as against standard waste-tyre disposal methods. Such evaluations may provide guidance for policymakers and assist in the development of improved tyre recycling policies and sustainable fuel production frameworks for the future [53].

3.3. Advantages and Challenges of Tyre Pyrolysis Oil in Homogeneous Charge Compression Ignition Engines

TPO holds proven potential as an alternative fuel for HCCI engines that is both environmentally friendly and economically sound. Its major strength lies in its potential to solve the worldwide waste-tyre disposal problem. The pyrolysis process allows waste tyres to be turned into liquid fuel, facilitating waste management while promoting the circular economy by eliminating landfill and burning processes. TPO can be produced using inexpensive feedstock that is readily available, so it is economically feasible to produce fuel in large amounts. It has a high calorific value comparable to conventional diesel, converting energy efficiently. Challenges include uneven chemical composition or the existence of impurities, which can interfere with the stability of combustion and increase emission levels. Technical advancements and improved fuel standardisation processes are needed in order to extract maximum advantage from use in current HCCI engines. With suitable engine adjustments and improved emission management approaches, TPO remains a very promising alternative fuel candidate [54,55].

TPO use entails several challenges, however. High sulphur content is one major concern, potentially resulting in high SO₂ emissions, air pollution and acid rain. Improved desulphurisation technologies are needed to help minimise the environmental consequences of TPO manufacture and burning. TPO can also contain certain heavy metals and PAHs, which can be harmful to the environment—this brings a need for high-efficiency filtering. Such pollutants may also lead to wear and tear of the engine and a reduction in performance. The second-most important problem is the diverse chemical structure of TPO, which can depend on the particular tyres processed and the conditions of pyrolysis. Such variability can cause unpredictable combustion behaviour and make it difficult to ensure optimised engine performance and emissions control in HCCI engines. These issues can be tackled by enhanced fuel processing and quality management and by the use of dynamically engine-tuned fuel, helping TPO achieve its full potential as a clean and efficient alternative fuel [56].

To this end, further research and development is required. The major concern should be enhancing the pyrolysis process to maximise the fuel quality, mitigate its variability and achieve more reliable combustion properties. Achieving advancements in filtration and purification methods is also of great importance in order to remove dangerous impurities. At the same time, research should aim further to optimise engine design and control mechanisms. HCCI engines require modification for TPO’s special chemical and physical characteristics. Furthermore, extensive LCAs should be conducted to assess TPO’s overall environmental impact from manufacture to burning, compared with conventional diesel fuel and other biofuels. Thus, TPO can become a feasible alternative fuel with the potential to contribute to waste-tyre management, promote energy mix and help reduce greenhouse gases [55,57].

4. Comparative Analysis of Hydrogen and Tyre Pyrolysis Oil in Homogeneous Charge Compression Ignition Engines

As a fuel choice for HCCI engines, hydrogen has several important advantages compared with TPO. Hydrogen’s high energy content and clean-burning nature with zero carbon emissions make it a strong option to reduce greenhouse gases and help improve air quality levels. Hydrogen’s wide flammability window offers greater flexibility in engine operation, which helps achieve higher efficiency even under varying engine loads. Its very fast flame speed supports fuller combustion, which may lead to a reduction in unburned hydrocarbons escaping from the exhaust [58].

However, huge challenges remain. Producing hydrogen, plus storing and distributing it, consumes considerable energy and remains expensive with present technology. High flammability, combined with risks of leakage, means that use of hydrogen demands strict safety protocols and specially designed storage equipment and handling systems. Because hydrogen has a low volumetric energy density, vehicles need either high-pressure storage tanks or cryogenic liquid systems, both of which add to design complexity and increase costs [31].

TPO, on the other hand, provides certain benefits when it comes to availability and cost feasibility, as it reuses waste tyres while using already-existing infrastructure for production and delivery. With an energy density higher than hydrogen, TPO also allows for easier storage and may offer a longer driving range for vehicles. However, the chemical makeup of TPO can vary widely depending on feedstock and processing conditions, which sometimes leads to unstable combustion and makes controlling emissions more difficult. While hydrogen combustion principally produces water vapour, TPO combustion creates multiple pollutants, such as sulphur oxides and PM, calling for more advanced after-treatment solutions to keep emissions under control [13,43].

Ultimately, the choice between hydrogen and TPO for use in HCCI engines will depend on developments in infrastructure, technology and environmental policies in the coming years. Hydrogen may prove more suitable in the long term, especially if renewable sources can be used to produce it. On the other hand, TPO promises a near-term solution for managing waste tyres and recovering energy. Future research should focus on making both fuel production and purification more efficient, along with improving HCCI engine designs so that they can handle these fuels better and extract maximum efficiency while keeping emissions as low as possible.

4.1. Engine Performance Metrics

Hydrogen and TPO exhibit different performance behaviours in HCCI engines. Hydrogen, because of its high energy content and very wide flammability range, helps improve engine efficiency under many different load conditions. But hydrogen’s very low volumetric energy density requires high-pressure storage tanks, adding complexity to vehicle design. TPO has a higher volumetric energy density, which allows for easier storage and increases vehicle driving range. However, TPO’s variable chemical composition and the possible presence of contaminants may lead to unstable engine performance and higher levels of emissions [59].

Comparing HCCI engine performance metrics, the differences are clear. Hydrogen usually achieves higher thermal efficiency, mostly because of its faster burning speed and wider operational window. Hydrogen combustion produces only water vapour, so CO₂ emissions remain very low. Under a high load, though, problems can emerge with NOx formation as the combustion temperature rises. TPO may generate more PM and sulphur oxides during combustion, requiring advanced after-treatment systems. TPO’s auto-ignition also tends to make it less stable than hydrogen, making combustion timing and steady operation more challenging to control in HCCI set-ups. For both fuels, due adjustment of factors such as injection timing, compression ratio and intake temperature remains important to maintain stable and efficient HCCI operation [60].

Choosing between hydrogen and TPO for HCCI application will usually be a matter of balancing performance results, emission outcomes and practical aspects of implementation. Hydrogen’s clean burning and potential to deliver zero-carbon operation make it a suitable option where strict emissions standards apply. However, the issues associated with production, storage and distribution continue to delay its wide-scale roll-out. TPO offers more immediate solutions, helping solve tyre-waste problems while facilitating energy recovery, sometimes at a lower overall system cost. Looking ahead, future research must stay focused on solving the specific technical challenges of both fuels—for example, improving hydrogen storage systems or refining TPO cleaning processes. Further advances in HCCI control systems and combustion chamber design may also help overcome current limitations, opening pathways to hybrid strategies that integrate the strengths of both fuel types [9,43,59].

4.2. Combustion Characteristics

The way hydrogen burns inside HCCI engines differs from conventional fuel combustion. Hydrogen’s high auto-ignition temperature and wide flammability limits mean that it can operate across a much broader range of engine conditions. Its rapid flame propagation helps speed up combustion, which may result in better thermal efficiency. However, its high reactivity sometimes creates issues, such as pre-ignition or knocking, which require careful management through the accurate control of injection timing and proper adjustment of the fuel–air mixture composition [61,62].

TPO exhibits different combustion features in HCCI engines because of its complex chemical makeup. TPO’s various hydrocarbon compounds lead to a multi-stage ignition process, which can influence the heat release rate and the timing of the combustion phases. Previous studies have indicated that TPO can be used successfully in HCCI engines, either by itself or blended with conventional diesel fuel [40,42]. Its high aromatic content can lead to higher PM emissions, but this issue can be controlled by optimising engine operating parameters and potentially also by blending TPO with hydrogen to form a hybrid fuel system that combines the strengths of both fuel types.

4.3. Emission Comparison

Hydrogen, a very clean-burning fuel, produces almost zero carbon emissions when used in HCCI engines. This property makes the use of hydrogen an attractive option for lessening greenhouse gases and supporting efforts against climate change. The exhaust from hydrogen combustion comprises mostly water vapour, along with some NOx. Since hydrogen fuel contains no carbon atoms, concerns over the production of CO₂, CO and PM, which normally occurs with fossil fuels, are automatically eliminated. However, hydrogen’s higher combustion temperature can result in increased NOx emissions [63]. These NOx gases pose a serious problem, potentially contributing to air pollution and smog formation. Researchers are exploring different methods to tackle this issue, including lean combustion techniques, EGR and more advanced control algorithms. With a combination of hydrogen and EGR in a DME-fuelled HCCI engine, research has shown that the load limit can be extended to 52% (24% to BMEP 3.62 bar), achieving up to 12% of hydrogen energy, with higher efficiency, no smoke and ultra-low NOx, CO, HC, and CO₂ emissions. This translates to a peak representable efficiency of 40%, with specific NOx emissions reduced to as low as 0.1 g/kWh [12,37].

TPO contains various hydrocarbons and impurities. When burned in HCCI engines, TPO usually produces higher levels of CO, unburned hydrocarbons and PM compared with hydrogen. This occurs mostly because of TPO’s complicated composition of hydrocarbon molecules, plus some contaminants from tyre manufacturing processes [42]. The determined performance and the emission characteristics of the combustion of a diesel engine working on a blend of tyre pyrolysis oil (TPO) and biodiesel injected using nano-additives were examined. The relationship between differing blending ratios and combustion characteristics and emission production was assessed. TPO was synthesised out of scrap tyres in the process of pyrolysis, and biodiesel was synthesised out of used cooking oil (UCO) using the transesterification method. A nanoparticle of strontium oxide (SrO) extracted from Moringaoleifera was added to the blends of the fuel because of its capacity to release oxygen to enhance combustion efficiency. Three mixtures of fuels were prepared in order to test them: B5TPO95SrO50: 5% biodiesel and 95% TPO with nanoparticles 50 ppm SrO. B10TPO90SrO100: 10% biodiesel, 90% TPO containing 100 ppm nanoparticles of SrO. And B50TPO50: 50/50 biodiesel/TPO without nano-additives. The blend B10TPO90SrO100 recorded the best performance in brake thermal efficiency (31.4) and brake-specific fuel consumption (0.21 kg/kWh) with a full engine load. The blend of B5TPO95SrO50 showed high levels of reduction in emissions, compared to conventional diesel, when the engine operated at the peak level of 27.9 Nm, i.e., a 2.05% reduction in unburned hydrocarbons (HC), 8.30 in carbon monoxide (CO), and 18.00 in nitrogen oxides (NOx). This data indicates that mixtures of blends of waste tyre pyrolysis oil and biodiesel based on UCO augmented with biogenic SrO nanoparticles provide a workable and feasible alternative fuel to diesel in the future [57].

Strategies can be put in place to improve TPO’s emission profile. For instance, blending TPO with other fuels, such as diesel or biodiesel, may improve its combustion behaviour and lower emissions. EGR can also help by reducing the combustion temperature, which limits NOx formation. More advanced fuel injection techniques and carefully optimised engine settings may lead to better combustion efficiency, further reducing CO and hydrocarbon emissions. Even with these improvements, however, TPO continues to produce higher emissions than hydrogen. But although hydrogen shows much better performance in terms of carbon-based emissions, its very high flammability leads to safety concerns over NOx emissions. Hydrogen’s fast flame speed and wide flammability window demand careful adaptation of engine design in order to prevent abnormal combustion events such as knocking or pre-ignition. Furthermore, hydrogen’s low density and very high diffusivity create complications for safe storage and transport, requiring specialised infrastructure and strong safety systems. Research examining HCCI combustion with and without 10% EGR of a modified single-cylinder diesel engine through the process of diesel vapour induction found that NOx emissions were reduced by 55% (without EGR) and 80% (with EGR), compared with conventional operation, due to improved mixture preparation and delayed ignition. Smoke emissions were reduced by 20% and 30% [6,64].

For both fuels, careful calibration and advanced control strategies are essential to achieve the best possible performance of the HCCI engine. With hydrogen, this will mainly involve very precise management of the air–fuel mixture ratio, injection timing and compression ratio so that the combustion phase is properly controlled while keeping NOx emissions low.

Although there are several benefits associated with homogeneous charge compression ignition (HCCI) engines, difficulties in governing the combustion phasing and emissions limit progress in the development of such engines. Design and validation of a Multi-Input Multi-Output (MIMO) controller of the HCCI engines capable of controlling the three most important issues, combustion phasing, engine load, and emissions, solely on load demand just as in the conventional engines. An Artificial Neural Network (ANN) based on a physics-based control model is able to predict optimal combustion phasing (CA50) and minimise emissions by employing the genetic algorithm with a multi-zone kinetic model. The validity of the controller is tested against experimental data in steady-state and transient cases. The results indicate that the controller has significant responses in terms of accuracy and speed, as it can follow load-based set-points and reject disturbances in 3–5 engine cycles with a maximum error of 0.04 bar (IMEP), 0.5 CAD (CA50), and 0.03 (emissions) uncertain elements; hence, its robustness and reliability in real-world HCCI [65].

In the case of TPO, approaches such as multiple injection events, variable valve timing and advanced after-treatment systems may be needed to handle its more complicated emission behaviour. Choosing between hydrogen and TPO for use in HCCI engines becomes a trade-off, balancing different emission patterns against the technical complexities of each fuel. Hydrogen produces almost zero carbon emissions but requires tight NOx control and strict safety measures. TPO allows for the recycling of waste tyres into fuel, but the presence of several pollutants carries its own challenges. Continued research into improving engine architecture, fuel treatment processes and emission control systems remains important to fully unlock the potential of both hydrogen and TPO fuels for HCCI engines and move closer to a cleaner transportation future. The study, under comparison, is that of energy, exergy, environmental, and enviro-economic output of diesel, waste tyre pyrolysis oil (WTPO), and oxyhydrogen (HHO) gas, used as a combination of fuel in dual-fuel compression ignition engines. The analysis has described a point to evaluate the thermal and exergetic efficiency; however, there is also the environmental impact of the integration of alternative fuels like WTPO and HHO along with the conventional diesel, which also has to be checked in terms of economic analysis.

First- (24%) and second-law (13%) efficiencies were the greatest using a combination of diesel + 10 L/min HHO. The 20% blend of WTPO recorded a decrease in environmental and enviro-economic impact of up to 9% [52]. The engine ran reliably with up to 90% DTPO, but performance at 100% DTPO was unacceptably poor. Blends of DTPO exhibited a maximum 3% reduction in the thermal economy, a 21% reduction in NOx and increased HC, CO and smoke. Delays in ignition by 2–2.5-degree CA were observed, with peak cylinder pressures elevated by 1.6 bar and 2 bar of DTPO80 and DTPO90 over the diesel fuel, respectively [66].

Table 3. Emissions comparison of hydrogen blends with TPO, diesel and petrol.

Emission Type	H2 + TPO Blend	H2 + Diesel Blend	H2 + Petrol Blend
CO2 emissions	Reduced compared with pure TPO; exact reduction varies based on the blend ratio and engine conditions, up to 18–22% at 30% H2 blend (avg.)	Decreased by approximately 10–15% compared with diesel alone	Reduced by approximately 11.7% on average; up to 31.2% reduction at a 20% H2 blend
CO emissions	Lower than TPO alone; specific values depend on the operating conditions, reduced to 20–35% at moderate engine load	Significantly reduced; exact figures vary across studies, ranging between 50 and 60% at a 30% H2 blend	Decreased with increasing H2 content; specific reductions depend on the blend ratio, up to 40% reduction at a 25% blend ratio
Hydrocarbon emissions	Decreased compared with TPO alone; specific values depend on the blend and engine load, normally in the range of 30–45%	Reduced by approximately 50% at a 30% H2 blend compared with diesel	Decreased with higher H2 content; exact reductions vary by up to 35–50% at a high H2 blend ratio
NOx emissions	Increased by 6–9% compared with TPO alone	Results vary: some studies report increases of up to 10%, while others report decreases or minimal changes	Variable (–5% to +12%); some studies indicate reductions while others show increases depending on the blend ratio and engine conditions
Smoke opacity	Reduced by up to 15–25% compared with TPO alone; specific values depend on the blend ratio up to 20–30%	Decreased from 67% (diesel) to 38% at a 30% H2 blend	Not specified in the available studies

Column No. 1 lists the types of emissions (CO₂, CO, HC, NOx, Smoke Opacity) that are compared across different hydrogen–fuel blends.

presents a comparison between hydrogen blends with TPO, diesel and petrol across multiple engine performance parameters.

4.4. Fuel Efficiency and Energy Density

Thanks to hydrogen’s very wide flammability range and extremely fast flame speed, combustion can stay efficient even under lean conditions. This can increase fuel efficiency, and emissions, especially NOx and PM, can be brought down significantly [8,37].

TPO, manufactured by breaking down waste tyres thermally, is another interesting option. Since TPO carries a high calorific value and contains many hydrocarbon compounds, it seems technically suitable for engine operation. TPO has the potential to improve the efficiency of HCCI engines, as its energy content stays higher than standard diesel fuels. However, because its composition is not uniform, varying depending on the pyrolysis process, maintaining stable and consistent combustion behaviour under HCCI conditions can sometimes pose difficulties [13,41,57].

When hydrogen and TPO are combined for HCCI combustion, tests have shown encouraging results for both fuel efficiency and energy density. The fast flame speed of hydrogen and its wide flammability limits may complement TPO’s high energy content, leading to improved combustion stability and potentially better engine performance. Adding hydrogen also seems to help balance some of the challenges caused by TPO’s varying composition. However, more research is needed to determine the blending ratios that work best while also addressing storage safety. A great understanding is required of the potential effects of such blended fuels on long-term engine durability and emission behaviour after extended operation.

4.5. Economic Feasibility and Scalability

HCCI engines have been receiving much attention lately because of their ability to deliver high efficiency and lower emissions compared with conventional engines. Researchers have been exploring the use of alternative fuels such as hydrogen and TPO to improve HCCI engines’ performance further while minimising their environmental impacts. Because of its high energy density and very clean combustion characteristics, hydrogen shows promising behaviour for use in HCCI systems. It offers a wide flammability range and rapid flame propagation, so combustion stays efficient even with lean fuel–air mixtures. This combustion behaviour leads to improved fuel efficiency while helping to reduce emissions, especially NOx and PM [67,68]. TPO, which is produced from the thermal decomposition of waste tyres through pyrolysis, also represents a potential alternative fuel option for HCCI engines. It carries a high calorific value and contains multiple hydrocarbon compounds, which make it suitable for combustion applications. When TPO is used for HCCI engines, it can sometimes improve fuel efficiency due to its higher energy content compared to conventional diesel fuel. However, since the composition of TPO often changes depending on the conditions of the pyrolysis process and the types of tyres used, there are also challenges in maintaining stable and repeatable HCCI combustion performance.

Blending hydrogen with TPO inside HCCI engines has shown promising results in terms of improving fuel efficiency and energy density. The fast flame speed and wide flammability range of hydrogen may complement the high energy content of TPO, potentially leading to better combustion stability and improved overall engine operation. The addition of hydrogen can help compensate for the composition variability of TPO fuel. However, further studies are needed to optimise the blending ratios while carefully addressing storage and safety concerns; it is also necessary to evaluate more fully how these fuel blends may affect engine durability over longer periods of use.

When considering the economic feasibility and scalability of the use of hydrogen and TPO for HCCI engines, several important factors are involved. Essential infrastructure for hydrogen production and distribution remains under development, which affects the cost-effectiveness and availability of the fuel in many regions. With the development of renewable energy sources, green hydrogen may become more affordable and widely accessible [55]. In the case of TPO, economic viability is strongly connected to the availability of waste tyres and the efficiency of the pyrolysis process. For TPO to scale up more efficiently, well-organised waste-tyre collection systems and more streamlined processing facilities are required. Although both fuels hold significant promise for future use in HCCI engines, more research and development is necessary to resolve technical barriers, fine-tune fuel-blending approaches and improve system efficiency towards achieving economic viability and wide-scale adoption.

5. Technological Advancements and Future Prospects

HCCI engines are high efficiency and low emission, and other fuel types such as hydrogen or waste tyre pyrolysis oil (TPO) have good prospects. The clean burning of hydrogen and high energy density enhance combustion and low emissions. TPO is a potentially good diesel alternative with high calorific value, but its unpredictable make-up can cause problems. The combination of hydrogen and TPO combustion greatly improves combustion stability and efficiencies, but further studies are required to achieve optimality in terms of ratios, safety and durability. Hydrogen is economically challenged to produce and to have supportive infrastructure, whereas efficient tyre waste processing will be the deciding factor between the viability of TPO. Both promising, the two types of fuel need additional research in order to implement them in practice [2,10,16,46,55,69].

5.1. Recent Developments in Hydrogen and Tyre Pyrolysis Oil Production

In recent years, hydrogen production has witnessed significant advancements towards more sustainable and cost-effective methods. Green hydrogen has attracted particular attention: it is produced through electrolysis, which uses renewable energy sources instead of fossil fuels. This makes it a much cleaner option when compared with hydrogen produced by conventional methods. Innovations in electrolysis technologies, such as proton exchange membrane electrolysers and solid oxide electrolysis cells, have helped improve process efficiency while also bringing down the cost of production. In addition, researchers have been exploring biological pathways for hydrogen generation, such as dark fermentation and photo fermentation, which may contribute to more sustainable hydrogen production in the future [70].

TPO production has also seen noticeable developments in recent years. By improving reactor designs and optimising process parameters, better yields and improved TPO quality are now being achieved. One such advancement is microwave-assisted pyrolysis, which has become a more energy-efficient alternative to older heating methods, offering faster processing and allowing for better control over the composition of products. Researchers have also been experimenting with catalytic pyrolysis techniques to help improve TPO quality, mainly by reducing sulphur content and increasing the fraction of valuable hydrocarbon compounds. Additionally, efforts to integrate pyrolysis with other waste treatment technologies, such as gasification and Fischer–Tropsch synthesis, have opened up new options for producing high-value fuels and chemicals directly from waste tyres [51,71].

More and more research is focusing on dealing with some of the technical challenges associated with the use of TPO, especially its high sulphur content and elevated viscosity. Advanced desulphurisation methods have been developed, such as oxidative desulphurisation and adsorptive desulphurisation, which help lower sulphur levels and make TPO a more acceptable option for engine and industrial use. Moreover, blending TPO with other fuels or using it as an additive in conventional fuels has been shown to be a promising innovation, both in terms of fuel performance and emission reduction [72].

5.2. Innovations in Homogeneous Charge Compression Ignition Engine Design for Alternative Fuels

Innovations in HCCI engine design have mostly been aimed at solving some of the problems associated with combustion control and expanding the operating window. One key development has been the introduction of advanced fuel injection strategies. Systems such as variable valve timing and variable valve lift have already been applied to HCCI engines and allow for much finer control over intake charge temperature and composition. This makes the auto-ignition process easier to manage, especially while using alternative fuels that behave differently in combustion when compared with normal gasoline or diesel [14,73].

Another major innovation in HCCI design has been the development of dual-fuel systems that allow engines to operate with more than one fuel type. Researchers have tested different set-ups, for example, where natural gas or hydrogen is added as a secondary fuel to help improve combustion stability when running engines on biofuels or other kinds of alternative fuels. These dual-fuel arrangements usually rely on advanced fuel delivery systems and special mixing techniques to maintain combustion across a wide range of engine speeds and loads. In addition, many set-ups employ advanced sensors and real-time control systems that continuously observe and adjust the combustion process as it occurs. This approach allows for very precise control over auto-ignition and how much heat is released during engine operation [15,16,18].

Thermal management solutions are also playing a critical part in improving how HCCI engines perform while using alternative fuels. Some engine designs now include completely new cooling systems, along with special insulation strategies, to keep cylinder temperatures in the sweet spot for efficient combustion. Some designs even use variable compression ratio mechanisms so that the engine can change its settings depending on the fuel type and actual running conditions. Finally, engineers have also started looking into the idea of integrating waste heat recovery technologies, including thermoelectric generators and organic Rankine cycle systems, which could help raise total engine efficiency even further when using alternative fuels [74,75].

5.3. Potential Synergies Between Hydrogen and Tyre Pyrolysis Oil

Hydrogen and TPO both offer interesting potential for synergistic use in the energy and transportation sectors. TPO, which is obtained from the thermal decomposition of waste tyres, carries a complex mix of hydrocarbons, while hydrogen is a clean-burning fuel known for its high energy density. The potential for synergy between these two energy sources will depend on the extent to which their individual properties are able to complement one another and the effective combination of production and utilisation processes [76].

One major area where synergy might be pursued is fuel blending and co-combustion. Hydrogen can be added as a secondary fuel when TPO is used in internal combustion engines, which can improve ignition properties, reduce emissions and increase total fuel efficiency. Blending can also help resolve some of the known challenges of TPO, such as its high viscosity and its tendency towards incomplete combustion under certain conditions [71].

Production processes may hold further potential for synergy. When tyres undergo pyrolysis, they yield oil but also generate syngas, which contains a significant portion of hydrogen. This hydrogen can be separated and purified to be used directly as a clean fuel or to serve various industrial needs. Additionally, the carbon-rich char that is a by-product of tyre pyrolysis can act as feedstock in gasification processes for producing additional hydrogen. Such an integrated production pathway promises to improve resource efficiency while reducing the total environmental footprint of both processes [22,46,71,76].

Besides these technical synergies, the combination of hydrogen and TPO technologies might also help push the development of more sustainable circular economy approaches. Waste tyres, which normally pose major environmental disposal challenges, can instead be converted into valuable energy products through pyrolysis. If hydrogen production and use are integrated into this system, the entire process might be made even more energy-efficient and environmentally friendly. This kind of synergistic framework closely complements worldwide goals of reducing waste, lowering dependence on fossil fuels and moving towards greener energy systems over the long term.

5.4. Research Gaps and Future Directions

Although research has shown the potential of the use of hydrogen and TPO in HCCI engines, several avenues remain to be explored. One major gap relates to the question of how to optimise the blending of hydrogen and TPO specifically for HCCI engine applications. Studies have already demonstrated the potential benefits of these fuel blends for HCCI engines, but a more comprehensive investigation is needed in order to establish optimal blend ratios, proper injection techniques and fine-tuned combustion parameters. Future work should aim at creating accurate fuel formulations that can help maximise engine performance and bring down emissions while ensuring stable HCCI combustion across various operating conditions and engine loads. Another important research gap is related to the long-term durability and reliability of HCCI engines when continuously operated with hydrogen and TPO blends. Most of the research conducted so far has focused on short-term performance testing and emission analysis, but data on the long-term effects on engine parts are limited. Future studies should examine how extended running with these fuels can affect different engine components, considering possible corrosion, material wear and unwanted deposit formation inside internal engine components. In addition, new research efforts are also needed to develop specialised engine materials and lubricants that can handle the specific challenges created by burning hydrogen and TPO fuels in HCCI combustion environments for long periods.

The integration of advanced control strategies and real-time combustion sensing technologies also appears to offer promising pathways for improving hydrogen- and TPO-fuelled HCCI engine performance. Since HCCI combustion is sensitive to changes in running conditions, developing robust control systems that can automatically adapt in response to variations in fuel composition, ambient temperature and engine load is extremely important. Future research might also explore how artificial intelligence and machine learning-based algorithms can be applied to optimise engine operation and emission management in real time, helping to make the wider commercial use of this technology more feasible and practical.

6. Environmental and Sustainability Implications

The use of hydrogen and TPO in HCCI engines involves important environmental and sustainability considerations. From an environmental perspective, both of these alternative fuels offer certain advantages towards reducing greenhouse gas emissions and improving overall air quality. Hydrogen is a clean-burning fuel whose main combustion product is water vapour; this means CO₂ emissions during engine operation are practically nil. TPO, which comes from waste tyres, provides a way to recycle a problematic waste stream into useful energy, helping to reduce the environmental burdens of tyre disposal and landfill problems. The combination of these fuels for HCCI engines can potentially lead to lower overall emissions of criteria pollutants, such as NOx and PM, compared with conventional diesel engines [52].

However, the environmental benefits of hydrogen and TPO for HCCI engines must be considered within the broader context of their production and supply chains. The sustainability of hydrogen as a fuel depends largely on its production method. While green hydrogen produced through electrolysis powered by renewable energy sources offers significant environmental advantages, currently, predominant production methods, such as steam methane reforming, still rely on fossil fuels and contribute to greenhouse gas emissions. Similarly, the production of TPO through pyrolysis requires energy input and may generate emissions, although these are generally offset by the benefits of waste-tyre recycling. Future research should focus on LCAs to quantify the net environmental impacts of these fuel systems, considering resource extraction, production, distribution and end-use emissions [29,54,55].

From a sustainability perspective, the use of hydrogen and TPO in HCCI engines is compatible with the circular economy and the broader shift towards more sustainable transport systems. TPO is essentially a mode of resource recovery, finding new uses for old material and reducing the need for fresh petroleum resources. Hydrogen, too, when made with renewable energy, can offer a route towards decarbonising the transportation sector. However, plenty of challenges remain, especially in building the proper infrastructure for storing and distributing hydrogen on a larger scale. As for TPO, its long-term sustainability depends on the continuing availability of waste tyres and on improvements to pyrolysis methods to deliver better yields while keeping the environmental downsides low. Moving ahead, research needs to focus on breaking down these barriers while also figuring out how hydrogen and TPO-powered HCCI engine systems can be fully integrated into larger sustainable energy systems. This might mean the incorporation of waste heat recovery technologies or the direct linking of fuel production processes with renewable energy sources to create a more complete and environmentally balanced energy loop.

6.1. Life-Cycle Assessments of Hydrogen and Tyre Pyrolysis Oil as Homogeneous Charge Compression Ignition Fuels

LCAs of the use of hydrogen and TPO in HCCI engines are a crucial means of evaluating the respective environmental impacts and long-term sustainability potentials of these fuels. Hydrogen, especially when produced through electrolysis relying on renewable energy sources, has strong potential for lowering greenhouse gas emissions. Its combustion creates only water vapour, eliminating CO₂ at the point of use. However, these environmental benefits depend on how the hydrogen is produced in the first place. Current mainstream production methods, such as steam methane reforming, continue to rely on fossil fuels and still release notable emissions into the atmosphere [25,77].

TPO produced from waste tyres is a potential means of addressing the problems related to tyre disposal while also supplying an alternative liquid fuel source. Through pyrolysis, waste tyres are broken down into oil that can be used in engines, helping to reduce landfill waste and demand for fresh petroleum resources. Energy consumption and emissions linked to the pyrolysis process must be included in a full LCA. Although the use of TPO in HCCI engines may help reduce some pollutants compared with conventional diesel engines, more detailed research is necessary to properly quantify these advantages across the entire fuel cycle [78].

LCAs need to include several stages, focusing on raw material extraction, actual fuel production, distribution networks and emissions generated during end-use combustion. For hydrogen, this means evaluating the impacts from different production techniques, alongside storage requirements and transportation challenges. For TPO, the assessment should include steps such as waste-tyre collection, pyrolysis plant operations and further refining stages if required. Both fuels promise to make meaningful contributions to circular economy models and more sustainable transportation systems, but multiple hurdles remain, especially in terms of building the proper infrastructure, ensuring scalability and securing long-term resource supplies, particularly in the case of TPO. Future research needs to focus on optimising fuel production technologies, improving engine combustion efficiency and finding ways to better integrate these fuel systems into larger sustainable energy frameworks [77,79].

6.2. Contributions to the Circular Economy and Waste Management

The combination of hydrogen and TPO in HCCI engines offers a promising direction for the development of the circular economy and waste management within the automotive sector. Hydrogen, when produced through sustainable processes such as renewable-energy electrolysis, can be a zero-emission fuel. Its use in HCCI engines may help diminish greenhouse gas emissions and contribute to better air quality, especially in heavily populated urban regions. However, these benefits are highly dependent on how the hydrogen is produced, highlighting the need for continuous investment in renewable energy systems that can support large-scale green hydrogen production [55,71,79].

TPO derived from waste tyres provides a double benefit: it helps address the growing problem of tyre disposal while supplying an alternative liquid fuel source. The pyrolysis process not only lessens the amount of waste sent to landfills but also recovers valuable energy resources from old tyres that would otherwise be discarded. TPO use in HCCI engines can potentially lead to lower emissions of some pollutants when compared with traditional diesel fuels. It can support the goals of the circular economy by turning waste into useful fuel. Nonetheless, the environmental impacts of pyrolysis operations, including their energy demands and emissions, must be carefully examined to ensure that the overall effect remains positive [80].

Combining hydrogen and TPO for use in HCCI engines presents an innovative pathway to improve engine efficiency while solving part of the waste management problem. This strategy could lead to cleaner and more sustainable transportation systems by lowering dependence on fossil fuels and reducing environmental damage from tyre waste [81]. Yet, making such systems work successfully will require multiple challenges to be overcome: efficient hydrogen storage and delivery systems need to be developed, pyrolysis production processes need to be optimised to minimise their environmental footprints, and HCCI engine technology needs to be modified so that it can handle these blended alternative fuels effectively. Future research should focus on improving every stage of this system, from waste-tyre collection to fuel production to engine performance. Thus, such systems will promise comprehensive contributions towards achieving the goals of the circular economy and reaching improved waste management solutions [13,78].

6.3. Potential Impacts on Greenhouse Gas Emission Reduction

Use of hydrogen and TPO in HCCI engines promises major reductions in greenhouse gas emissions when compared with engines running on conventional fossil fuels. HCCI engines work by auto-igniting a premixed air–fuel mixture, which allows for high thermal efficiency while keeping emissions relatively low. Introducing hydrogen and TPO as alternative fuels aims to address not only environmental challenges but also waste management problems linked with tyre disposal [54].

Hydrogen burns cleanly and produces no CO₂ during combustion. Hydrogen can eliminate carbon-based emissions in HCCI engines, making an important contribution to reducing greenhouse gas levels. Because of hydrogen’s high flame speed and very wide flammability range, it is well suited for HCCI operation, supporting lean combustion while helping to increase fuel efficiency. However, some technical challenges need to be solved before wider adoption becomes possible, such as setting up safe storage systems, establishing the proper distribution infrastructure and dealing with NOx emissions, which tend to arise when the engine operates at higher loads [2].

Use of both hydrogen and TPO in HCCI engines has the potential to deliver significant cuts in greenhouse gas emissions compared with engines that run on regular fossil fuels. HCCI engines work through the auto-ignition of a premixed air–fuel mixture, allowing for higher thermal efficiency while keeping emissions low. In bringing hydrogen and TPO together, researchers are seeking to address not only environmental concerns but also waste management issues related to the increasing number of tyres being disposed of worldwide [55].

Hydrogen burns very cleanly, emitting no CO₂ during combustion. As a fuel in HCCI engines, it offers a pathway towards eliminating carbon-based emissions and towards an overall reduction in greenhouse gases. Hydrogen’s high flame speed and wide flammability range make it well-suited for HCCI operation because it supports lean combustion and helps improve overall fuel efficiency. Nonetheless, certain challenges need to be addressed to enable more widespread adoption, such as how to store hydrogen safely, how to build distribution networks and how to handle NOx emissions that result from higher engine loads [2].

TPO, which is produced through the thermal decomposition of waste tyres, brings a dual advantage, reducing the issues associated with tyre disposal while producing an alternative liquid fuel. TPO can replace conventional diesel in HCCI engines and help lower their carbon footprint, reducing landfill tyre waste and lowering fossil fuel dependency. It can also help decrease PM emissions while shifting consumption away from petroleum-based fuels. However, due to variations in TPO’s chemical composition and its sulphur content, additional fuel treatment or blending with other fuels may be necessary to achieve stable engine performance and acceptable emission levels [81].

When hydrogen and TPO are combined for use in HCCI engines, they offer opportunities for synergistic improvements. The high reactivity of hydrogen can help offset TPO’s lower cetane number, leading to better combustion stability and allowing the engine to operate across a wider range of conditions. Such dual-fuel solutions may provide even greater reductions in greenhouse gas emissions than either fuel can deliver on its own. Besides the technical gains, integration also conforms with circular economy models by encouraging the use of waste-derived fuels while reducing the overall carbon emissions of transportation systems.

7. Regulatory Framework and Policy Considerations

Regulatory frameworks dealing with the use of hydrogen and TPO in HCCI engines are still taking shape as these alternative fuels slowly gain attention in the automotive sector. Hydrogen, a clean-burning fuel that produces zero carbon emissions during combustion, faces very strict safety rules, mostly because of its high flammability and the risk of leakage. In many countries, governments are currently working on creating detailed standards that cover how hydrogen should be stored, transported and safely used by vehicles. These rules aim to protect public safety while also keeping environmental concerns in check. In the case of TPO, regulations are focused more on the production stage, since pyrolysis operations can release harmful pollutants if not properly managed. Environmental agencies continue to develop guidelines for safe production and fuel quality standards that predicate TPO use in engines [54].

Policies around the use of hydrogen and TPO in HCCI engines cover areas such as economic support, infrastructure building and emission control rules. Various policymakers are studying the potential of tax breaks and government subsidies to help speed up the production and use of these fuels in practice. Significant investment is being considered to establish necessary hydrogen refuelling stations. As for TPO, various policies are being developed to promote the goals of the circular economy by supporting the recycling of old tyres into usable fuel. Meanwhile, existing emission regulations are being adjusted to accommodate the combustion behaviour of HCCI engines running on hydrogen or TPO, with a particular focus on reducing NOx and PM emissions.

Several challenges remain in the implementation of these frameworks and policies. One main difficulty is finding the right balance between establishing strict safety demands and allowing enough room for technical innovation. Furthermore, different regions are at various stages of technological readiness, complicating uniform policymaking. To make possible the more widespread use of hydrogen and TPO in HCCI engines, it is essential to harmonise standards across countries and legal systems. Policymakers also need to take into account the full life-cycle emissions of these fuels, including the impacts of their production and distribution chains. As research continues and technologies improve, regulations and policies will need to remain flexible for adjustment in tune with new data and technical breakthroughs [82].

7.1. Current Regulations on Alternative Fuels in Transportation

Regulatory frameworks governing hydrogen and TPO use in HCCI engines are still under development. Hydrogen, a clean-burning fuel that generates no carbon emissions during combustion, is subject to very strict safety rules, mainly because of its highly flammable nature and leak risks. Governments in many parts of the world are now working on creating specific standards that deal with the storage, transportation and safe vehicle use of hydrogen. The focus of these efforts is on ensuring public safety while also protecting the environment. In the case of TPO, the regulations tend to highlight the production process because the pyrolysis operation may release dangerous emissions if not controlled correctly. Environmental bodies are continuing to work on guidelines for safe TPO manufacturing and fuel quality so that TPO can be approved for engine use [83].

Policies promoting the use of hydrogen and TPO in HCCI engines bring together several elements, including financial incentives, infrastructure construction and emissions standards. Policymakers are examining various options, such as tax breaks, subsidies and financial support, to encourage the production and market use of these fuels. Investment is under discussion for expanding the hydrogen fuelling networks that will be necessary for large-scale adoption. For TPO, policy is aimed more towards supporting circular economy practices by encouraging tyre recycling in fuel production. In addition, current emission regulations are being revised to better reflect how HCCI engines behave when running on hydrogen or TPO, and special attention is being given to lowering NOx and PM emissions [84,85].

The process of putting these regulatory systems and policies in place faces several hurdles. Balancing strict safety rules with the need to allow room for technical advancements remains a major issue. Different regions and countries have varied levels of technological advancement, creating discontinuities. To allow for the smooth global adoption of hydrogen- and TPO-powered HCCI engines, standards across jurisdictions need to be better harmonised. At the same time, policymakers must not ignore the full-life-cycle emissions of both fuels, observing not only engine operation but also fuel production, transportation and distribution. Only in this way can emission reduction targets be met. As research continues to advance and technology evolves, policy frameworks will need to remain flexible and adapt to new developments.

7.2. Incentives and Barriers for the Adoption of Sustainable Homogeneous Charge Compression Ignition Fuels

The push to embrace sustainable fuels in HCCI engines, especially hydrogen and TPO, emerges from growing environmental concerns in connection with wider efforts to shift towards cleaner energy systems. Governments in many countries are now introducing tax credits, subsidies and grants to boost the production and practical use of these fuels. Most of the financial support for hydrogen use targets infrastructure development, such as the construction of refuelling stations, to help grow the hydrogen-powered vehicle sector. In the case of TPO, incentives often aim to promote circular economy models by encouraging the recycling of waste tyres into fuel sources. Carbon pricing mechanisms and emission trading schemes may also help to make these fuels more economically competitive in comparison with regular fossil fuels [10].

Numerous barriers to the large-scale use of hydrogen and TPO in HCCI engine applications remain. Hydrogen, for example, faces the problem of high costs connected to production, storage and transportation. The lack of proper refuelling stations prevents any users from making the switch. Hydrogen is also highly flammable, and the risk of leaks gives rise to safety concerns and necessitates strict systems for handling, adding to overall costs.

TPO brings its own challenges. The main issue is the inconsistency of fuel quality, since different feedstocks create variations in chemical composition. Contaminants are sometimes detrimental to engine performance, and producing and processing TPO usually requires dedicated equipment, incurring higher expenses. In both cases, there are also technological obstacles. HCCI engines running on hydrogen or TPO usually need advanced systems that can control combustion timing and manage flame behaviour, making these engines more complicated than standard diesel alternatives. The long-term durability of engines when using these fuels continuously is also not yet fully understood; higher maintenance costs or shorter engine lives may become factors to take into consideration. Efforts to improve HCCI engines are ongoing, working to ensure that they run reliably, efficiently and stably on hydrogen or TPO across varying loads and environmental conditions. Policy uncertainty is another factor. Since the regulations and emission rules for hydrogen and TPO keep changing, many industries and investors hesitate to commit to these technologies fully. The absence of harmonised standards across different countries or regions adds one more layer of complexity that slows global adoption. In addition, full-life-cycle emissions for both fuels still require thorough evaluation, including how these fuels are produced, transported and delivered. These studies tend to be complex and time-consuming, which can further delay decision-making.

7.3. Policy Recommendations for Promoting Hydrogen and Tyre Pyrolysis Oil Adoption

Policies that aim to promote hydrogen and TPO adoption should focus on removing the main barriers to this process while also pushing the incentives that already exist. Hydrogen policymakers need to prioritise investment in infrastructure, especially networks of hydrogen refuelling stations. Much of this development can occur through public–private partnerships, combined with targeted financial support programmes. Research and development grants for projects working on improving hydrogen production methods and storage systems must be given higher priority so that production costs can be brought down while improving safety levels. Creating common safety standards and working towards harmonised regulations across different regions may ease public safety worries and accelerate acceptance of hydrogen technology. As to TPO, policymakers should focus on developing quality standards to help ensure that the fuel stays stable, both chemically and performance-wise, while in use in engines. At the same time, governments can offer incentives that allow tyre recycling facilities to upgrade their pyrolysis equipment, which may help them produce better-quality TPO fuels. Governments can also launch extended producer responsibility schemes to encourage tyre manufacturers to take larger roles in supporting recycling programmes and helping expand TPO fuel production capacities.

On the technology side, more research funding must go into studies focused on optimising HCCI engines for handling a combination of hydrogen and TPO. Special focus must be given to improving combustion control systems while avoiding negative impacts on engine durability over the long term.

Policymakers should focus on preparing clearer long-term roadmaps for the adoption of alternative fuels, which would result in better predictability for investors and regular users. This could involve setting targets for how much hydrogen or TPO needs to be integrated into the transportation or industrial sectors over the next years, supported by phased updates to emissions rules in response to ongoing developments. Offering financial perks, such as tax credits on vehicles or machinery running on these fuels, may also help boost demand. Full-life-cycle emission evaluations can be directly implemented in policy design so that hydrogen and TPO adoption remain properly aligned with larger climate and environmental ambitions. Policymakers can thus create much stronger support structures, accelerating the implementation of hydrogen and TPO usage in HCCI engines and driving a cleaner and more sustainable energy system for the future.

8. Conclusions

This review article has explored the potential of hydrogen and TPO for use in HCCI engines. Several key points can be highlighted in conclusion. Hydrogen, with its very high energy density and clean burning, as well as its fast flame speed and broad flammability limits, can boost engine efficiency while reducing emissions. Despite these benefits, real-world use still faces hurdles: storage remains a complicated question, transport logistics solutions are not yet fully developed, and necessary infrastructure is widely lacking. TPO, manufactured from waste tyres, not only acts as an alternative fuel source but also helps address global waste management problems. The performance of TPO in HCCI engines almost equals that of diesel, while emissions, such as PM and NOx, are significantly lower. However, one major issue with TPO is its irregular chemical makeup, which sometimes leads to higher pollutant outputs. There is still a need for improved fuel refinement and standardisation processes.

• Each of the two fuels has particular merits and challenges that make them interesting options in the transition to cleaner combustion technologies.
• Hydrogen provides extremely high energy density, combustion without carbon emissions, rapid flame speed and a broad flammability range, which are all factors promoting greater efficiency and lower emissions in HCCI engines. Nevertheless, its use in the real world is restricted by the need for elaborate storage solutions, inadequate transport logistics solutions and the wide lack of refuelling infrastructure.
• TPO, obtained through waste-tyre pyrolysis, simultaneously addresses demands for alternative fuel and waste management solutions. TPO is capable of providing a performance comparable to diesel in HCCI engines while reducing emissions, especially PM and NOx. However, variable chemical composition may lead to unpredictable combustion and pollutant production. Improved standardisation and refining processes are needed in order to ensure fuel quality.
• Hydrogen fuel optimisation will require accurate mixture preparation, proper timing of auto-ignition and a controlled release rate to avoid engine knocking. TPO optimisation will require better fuel injection and EGR methods and fine-tuning of combustion timing.
• Combining hydrogen and TPO in a dual-fuel system could help counteract the shortcomings of both fuels. Such blends might combine the volatile nature of TPO combustion with the consistent and clean-burning nature of hydrogen, increasing combustion stability, improving efficiency and reducing emissions.
• Further research and development is required to maximise the potential of hydrogen and TPO use in HCCI engines. Above all, improvements are required in fuel production technologies and engine design. More effective technologies should also be developed to help control emissions. Such advancements will help eliminate the existing constraints in fuel scalability, engine performance and infrastructure preparedness.
• As systems with high intrinsic thermal efficiency and low emissions, HCCI engines provide an ideal platform to introduce sustainable fuels such as hydrogen, TPO, biofuels and waste-based synthetic fuels. These fuels can help achieve a cleaner and more secure energy future in the transportation sector alongside ongoing policy support, technological innovation and investment in research.