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Article

Evaluation of Tire Pyrolysis Oil–HVO Blends as Alternative Diesel Fuels: Lubricity, Engine Performance, and Emission Impacts

by
Tomas Mickevičius
1,
Agnieszka Dudziak
2,*,
Jonas Matijošius
3,* and
Alfredas Rimkus
4
1
Department of Mechanical, Energy and Biotechnology Engineering, Vytautas Magnus University Agriculture Academy, Universiteto St. 10, Akademija, LT-53361 Kauno, Lithuania
2
Department of Power Engineering and Transportation, Faculty of Production Engineering, University of Life Sciences in Lublin, 20-612 Lublin, Poland
3
Mechanical Science Institute, Vilnius Gediminas Technical University-VILNIUS TECH, Plytines Str. 25, LT-10105 Vilnius, Lithuania
4
Department of Automobile Engineering, Faculty of Transport Engineering, Vilnius Gediminas Technical University-VILNIUS TECH, Plytinės Str. 25, LT-10105 Vilnius, Lithuania
*
Authors to whom correspondence should be addressed.
Energies 2025, 18(16), 4389; https://doi.org/10.3390/en18164389
Submission received: 15 July 2025 / Revised: 7 August 2025 / Accepted: 13 August 2025 / Published: 18 August 2025
(This article belongs to the Special Issue Future Perspectives of Internal Combustion Engines of High Efficiency: Analysis, Modeling and Control Strategies and Application of Sustainable Fuels)
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Abstract

In the pursuit of sustainable and circular energy sources, this study examines the potential of tire pyrolysis oil (TPO) as a diesel fuel substitute when combined with hydrotreated vegetable oil (HVO), a second-generation biofuel. At varying TPO-HVO blend percentages, this investigation evaluates engine performance and emissions in relation to critical fuel parameters, including density, viscosity, and lubricity. The high-frequency reciprocating rig (HFRR) method was employed to examine tribological aspects, and a single-cylinder diesel engine was tested under various load conditions. The findings indicated that blends containing up to 30% TPO maintained sufficient lubrication and engine performance to comply with diesel standards, concurrently reducing carbon monoxide and smoke emissions. The increase in TPO proportion resulted in a decrease in cetane number, an increase in NOx emissions, and a rise in viscosity, particularly under full engine load conditions. The utilization of TPO is crucial for converting tire waste into fuel, as it mitigates the accumulation of tire waste and reduces dependence on fossil fuels, despite existing challenges. This study provides critical insights into the efficacy of blending methods and underscores the necessity of additional fuel refining processes, such as cetane enhancement and desulfurization, to facilitate their integration into transportation energy systems.

1. Introduction

Recently, the world has been experiencing rapid growth in energy demand, especially in the transport sector, where the number of vehicles and energy consumption are increasing [1]. At the same time, the intensive use of fossil fuels increases the concentration of greenhouse gases in the atmosphere, contributing to climate change. In order to achieve the goals of the Paris Agreement and reduce CO2 emissions, radical changes are necessary in the energy sector [2,3,4]. Consequently, the need and relevance of renewable and alternative fuels are rising since they are regarded as one of the key answers for lessening the environmental effect of transport pollution. Many nations are tightening rules on harmful emissions in line with the rising need for alternative fuels as well as related issues. The European Union (EU) started to implement ever more rigorous vehicle emission standards [5]. The Euro 6 standard, which has been in place since 2014, for instance, has greatly lowered NOx and particle emissions from diesel engines in comparison to earlier phases [6,7]. Such ever more strict environmental criteria are motivating the car sector to use more sophisticated engine technologies [8] and change to cleaner fuels [9,10].
One of the growing environmental problems is the accumulation of waste, especially car tires. Tire pyrolysis oil is similar in its properties to petroleum-based fuels [11]. Only a small part of them (about 15–20%) are reused or renewed by retreading, while the rest goes to landfills or is illegally dumped into the environment, causing long-term pollution [12,13]. One of the most promising ways to address the problem of tire waste is to convert this waste into a useful fuel by pyrolysis [14,15]. Tire pyrolysis is a thermochemical process in which tires are decomposed at high temperatures (typically 400–600 °C) in the absence of oxygen (inert atmosphere). During pyrolysis, complex hydrocarbon polymers (the composition of tire rubber) are broken down into smaller molecules, yielding three main products: liquid oil, solid residue product (carbon char and soot), and flammable gases [16]. The liquid product obtained during this process is called tire pyrolysis oil (TPO)—a dark brown, strongly odorous liquid with significant energy value. Typically, about ~40–55% oil (by weight) can be recovered from tire pyrolysis, depending on the process conditions, the rest being solid products and gases. Studies show that pyrolysis is currently considered one of the most effective ways to manage tire waste, as it allows not only to reduce the volume of waste but also to recover part of the energy stored in them [17].
However, despite the listed disadvantages, tire pyrolysis oil has great potential as an alternative fuel. Many scientists claim that TPO could be successfully used as a diesel engine fuel, especially if it is not used pure but mixed with other fuels or processed to improve its properties [11,18]. The energy value of TPO is similar to that of diesel, so it can provide sufficient power to the engine. In addition, the use of TPO would help solve the waste problem—it is a great example of a circular economy, when waste is converted into resources. According to Kumaravel et al. [18], recycling tires into fuel not only creates a new fuel source that promotes more sustainable production and consumption, but it also helps to reduce environmental waste. TPO is also known for its high concentration of aromatic compounds with a high octane rating; thus, adding TPO may improve the characteristics of gasoline or act as a diesel fuel addition to increase energy density. Also, the oxygen compounds present in tire oil (if formed by tire rubber additives during pyrolysis) can lead to a slightly oxygen-enriched fuel, which produces less soot when burned. In summary, tire pyrolysis oil is a valuable secondary product, the proper use of which could contribute both to the enrichment of alternative fuel resources and to solving the waste problem [19].
Due to the limitations of pure TPO, practical studies have mostly focused on blending TPO with other fuels, such as diesel or biofuels, to exploit the energy potential of TPO while mitigating its undesirable properties. Over the past two decades, a number of experimental and review studies have been conducted to evaluate the effects of various TPO blends on engine performance and emissions. Most studies show that small amounts of TPO can be incorporated into diesel without significant design changes. For example, Haseeb Yaqoob and colleagues [11] reviewed a number of studies and concluded that TPO concentrations of up to ~10–20% in a diesel blend do not typically cause significant problems in combustion or engine stability. Blends with a small proportion of TPO burn similarly to pure diesel, as most of the properties (cetane number, evaporation) are still provided by the base fuel. In addition, such blends usually have a sufficiently high cetane number, so the ignition delay is not significantly extended. The literature indicates that when using TPO–diesel blends, engine power and efficiency can even increase slightly, especially at optimal mixing proportions.
However, as the proportion of TPO in the mixture increases, the impact on engine performance and emissions becomes increasingly noticeable. At higher TPO contents (e.g., 30–50% and above), the decrease in cetane number leads to a longer combustion delay, which allows combustion to occur more rapidly and at higher temperatures. As a result, an increase in NOx emissions is usually observed—in some cases, it has been recorded that nitrogen oxide emissions increase when using mixtures with a higher proportion of TPO compared to pure diesel. On the other hand, particulate matter (PM) and CO emissions often decrease with increasing TPO content, as TPO contains some oxygen and less saturated (paraffinic) components than diesel. Yaqoob et al. [11] summarized that two trends have been observed in the literature: some studies showed higher NOx emissions with TPO, while others, on the contrary, showed lower NOx emissions; however, in almost all cases, CO, CO2, and PM emissions tended to decrease with TPO due to a better oxidation process. For example, fuels containing TPO generate less soot, so the oxidation of some soot to CO2 improves, and accordingly, CO and soot emissions are reduced. One study indicated that when ~10% TPO was added, smoke was significantly reduced, and CO emissions were reduced due to the residual oxygen in the TPO composition [11]. However, changes in NOx are ambiguous: some authors found that NOx increases due to improved combustion (more oxygen, hotter combustion), while others found that due to a longer ignition delay, some combustion actually occurs later and the peak temperature is lower, so NOx decreases. Therefore, there is no complete consensus in the literature on the effect of TPO on NOx emissions—it depends on the type of engine, fuel delivery characteristics, and even variations in the composition of TPO [1].
In addition to diesel, TPO has also been blended with other biofuels. Researchers have tried to create blends where the base fuel is second-generation biodiesel (e.g., the same HVO or FAME biodiesel) with the addition of TPO. Such a combination is interesting in that two completely alternative fuels without a fossil component are blended. For example, one study examined a blend of HVO fuel with 15%, 30%, and 60% TPO—the results obtained showed that with increasing TPO content, engine power slightly decreases due to poorer combustion properties, but in terms of emissions, in certain cases, pollution does not exceed the standards. This suggests that with a properly balanced composition, even a high proportion of TPO can be used in fuels. It should be noted that research into alternative fuels is not limited to TPO. This shows that the scientific community is looking for complex solutions—combining different types of alternative fuels to make diesel engines operate as cleanly and efficiently as possible. In this context, TPO research is part of a broader effort to diversify fuel sources and reduce fossil fuel consumption.
Finally, fuel treatment technologies also require improvement, taking into account environmental requirements. For example, if TPO fuel is confirmed to be economically viable, the issue of sulfur removal (to ensure that SOx emissions comply with standards) should be addressed. Possible solutions—catalytic hydrodesulfurization or oxidative desulfurization—should be tailored specifically to TPO, as its composition differs from petroleum diesel. The pyrolysis process should also be optimized to obtain an oil with the most suitable composition (e.g., with a higher cetane number and fewer tarry substances). A techno-economic analysis [17,20] shows that the tire pyrolysis process can be economically justified if the recovered oil finds stable application as a fuel. Therefore, the scientific community is tasked with providing comprehensive data substantiating the efficiency and safety of TPO use.
In conclusion, tire pyrolysis oil research is a very promising but not yet fully explored area. In order to fully include TPO in the alternative fuel portfolio, it is necessary to fill the knowledge gaps—to study the lubricity properties with increasing TPO concentration, to detail the impact on engine operating parameters under various conditions, and to conduct additional experiments on long-term reliability. Only by addressing these issues in a comprehensive manner will it be possible to provide recommendations for the industrial use of TPO fuel. In this context, the literature examined reveals a solid foundation—global trends encourage the search for such solutions, the decline in oil resources forces them to be found, and the research of predecessors shows that TPO can become a real alternative; it is only necessary to solve the remaining problems by scientific and engineering means.
The scientific originality of this work is in its combined approach to blending tire pyrolysis oil (TPO) with second-generation biofuels (HVO) and concurrently assessing fuel lubricity (using the HFRR technique), physicochemical qualities, engine performance, and emissions. Although earlier TPO studies tend to emphasize short-term performance or emission criteria, very few of them thoroughly investigate tribological characteristics. This research examines the influence of varying TPO percentages in HVO-based mixtures on engine efficiency and mechanical wear, providing valuable insights for the use of elevated TPO concentrations while adhering to fuel standards.
This paper investigates the potential for synergy between two unconventional fuels—TPO derived from waste and HVO sourced from renewables—to mitigate environmental contamination by reducing waste and reducing reliance on fossil-based diesel. The findings offer valuable guidance on the optimal TPO ratio for attaining a balance between lubricity, power generation, and emission control, thereby promoting a more circular and sustainable transportation energy framework.

2. Materials and Methods

Tire pyrolysis oil is similar in its properties to petroleum-based fuels—it contains many hydrocarbons with a carbon-to-hydrogen ratio similar to diesel or gasoline components [21]. The calorific value (lower heating value) of TPO often reaches ~40 MJ/kg, i.e., similar to or slightly lower than that of conventional diesel fuel. The density of TPO is usually slightly higher than that of diesel, and the viscosity at room temperature is also higher, although it strongly depends on temperature. The chemical composition consists of various fraction components: aliphatic and aromatic hydrocarbons and cyclic compounds. A characteristic feature is a relatively high content of aromatic compounds and polycyclic aromatics, which leads to a high content of saturated hydrocarbons and a significant decrease in the cetane number. In fact, the cetane number of pure tire pyrolysis oil is often low (often indicated at about 30–40, when diesel is usually >50), which is why the ignition properties of such a fuel are worse. TPO also often has a high sulfur content (due to sulfur additives in the composition of tire rubber), which can reach even several percent—well above the limits of modern road fuel standards (ultra-low sulfur diesel is allowed <10 ppm sulfur). Viscosity is another challenge: the viscosity of TPO at 40 °C can be significantly higher than that of diesel (e.g., >5 mm2/s, when diesel is ~2–4 mm2/s), which makes it more difficult to ensure good spray quality in the injectors. Due to these characteristics, the use of pure tire pyrolysis oil in the engine causes certain problems [22]. As Yaqoob et al. point out, TPO has a low cetane number, high sulfur content, and high viscosity—these disadvantages limit its direct use in diesel engines [11]. A low cetane number means slower self-ignition of the fuel during compression, which can lead to longer ignition delay, increased risk of knock (detonation), and higher NOx formation due to higher combustion temperature. High sulfur content leads to higher sulfur oxide emissions and the risk of acid rain, and sulfur compounds in the engine can promote corrosion. Meanwhile, too high viscosity impairs fuel atomization—injectors form larger droplets that are unevenly distributed in the combustion chamber, resulting in incomplete combustion, more particulate matter, soot, and carbon monoxide. Viscosity also increases the load on the fuel pump. In summary, TPO requires some processing or blending to bring its properties closer to the requirements of standard fuels.
At Vytautas Magnus University, Faculty of Forestry and Ecology, Department of Environment and Ecology, Laboratory of Chemical and Biochemical Research of Environmental Technology, the density and kinematic viscosity of fuels and their mixtures of different compositions were studied. The Anton Paar device SVM 3000 (Anton Paar GmbH, Graz, Austria) was used to determine the physical properties of fuels (measurement errors 0.0002 g/cm3 and 0.1%, respectively).
Six fuel mixtures of different compositions were used in this study, the composition of which is as follows:
  • A total of 100% HVO—diesel fuel obtained by hydrogenation from vegetable oils and animal fats, wood, and industrial waste (HVO);
  • A total of 100% tire pyrolysis oil (TPO);
  • A total of 15% HVO and 85% tire pyrolysis oil (TPO15);
  • A total of 30% HVO and 70% tire pyrolysis oil (TPO30);
  • A total of 60% HVO and 40% tire pyrolysis oil (TPO60);
  • DD—mineral diesel.
The physicochemical properties of the pure fuels used are presented in Table 1.
Experimental studies of fuel lubricity were conducted in the tribology testing laboratory of the Vytautas Magnus University, Faculty of Engineering, Department of Mechanical, Energy and Biotechnology Engineering, Academy of Agriculture.
These studies were conducted with an HFFR high-frequency sliding motion stand. Fuel lubricity was determined by the wear trace formed on a rotating ball due to contact with a stationary plate immersed in fuel. The test was performed according to the international standard ISO 12156 (ASTM D6079-99 standard). The test conditions were as follows: an applied load of 200 g, stroke length of 1.0 mm, frequency of 50 Hz, and total test duration of 75 min. The fuel temperature during the test was maintained at 60 °C. Each test was repeated three times to ensure repeatability. According to the EN 590 standard, which defines the quality requirements for diesel fuel in the European Union, parameters such as density, viscosity, lubricity, cetane number, and sulfur content must meet specified limits to ensure fuel compatibility with diesel engines and environmental standards.
Images of ball wear traces were obtained using an optical microscope, “Nikon Elipse MA100” (Nikon Instruments Inc., Melville, NY, USA). The obtained images were zoomed in 200 times. The Excel 2016 program from the standard MS Office package was used to process the obtained experimental data. The wear scar on the ball was elliptical in shape, and both the major (a) and minor (b) diameters were measured using an optical microscope. The average wear diameter was then calculated as (a + b)/2, in accordance with standard HFRR evaluation procedures.
Experimental studies were carried out in the engine-testing laboratory of the Department of Mechanical, Energy and Biotechnology Engineering, Faculty of Engineering, Vytautas Magnus University, Academy of Agriculture. A direct injection, single-cylinder diesel engine “ORUVA F1L511” (Shijiazhuang Houfeng Trading Co., Ltd., Shijiazhuang, China) was used for these studies. During this study, the following were measured: engine fuel consumption, air consumption, torque, exhaust emissions, and smoke. The tests were performed at a constant crankshaft speed of 2000 min−1 under three load conditions: 10% load (4.5 N·m), 50% load (22.5 N·m), and 100% full load (45 N·m). These values represent the torque applied to the engine during each test cycle and were directly measured using a calibrated torque meter.
The volumetric air consumption was measured with a turbine gas meter, “Gazomierz Turbinowy CGT-02” (COMMON S.A., Łódź, Poland). Fuel consumption was measured by weighing with the electronic scale “SK-1000” and recording the time during which the engine consumed 50 g of fuel. In order to assess the influence of second-generation biofuels (HVO), used pyrolysis oil (TPO), and their mixtures on engine performance and exhaust emission indicators, TPO and HVO mixtures were prepared. The fuel mixtures used were mixed by volume. The fuels and their mixtures used in the experimental studies are presented in Table 2. The physical and chemical properties of the fuels used are presented in Table 3.
A Testo 350 XL flue gas analyzer (Testo SE & Co. KGaA, Lenzkirch, Germany) was used to measure carbon monoxide (CO), nitrogen monoxide (NO), nitrogen dioxide (NO2), and nitrogen oxide (NOX) emissions. The smoke content of the flue gas was measured with a Bosch RTT 100/RTT 110 flue gas opacity meter (Robert Bosch Corporation, Broadview, IL, USA).

3. Results

3.1. Influence of TPO–HVO Mixtures on Density, Viscosity, and Lubricity

Diesel quality requirements in Europe are set by the EN 590 standard. The most important indicators are density, viscosity, lubricity, cetane number, cetane index, boiling range, filtration limit, flash point, sulfur content, coking tendency, total contamination, and water content. When using fuels of different compositions and their mixtures, the properties of the fuels used change. The energy content per unit volume of fuel increases with increasing density. When using fuels with a higher density, which depends on the type of fuel, engine power and smoke increase. Therefore, it is required that the fuel density does not change much depending on the type of fuel (Robert Bosch GmbH., Gerlingen, German, 2004).
The addition of tire pyrolysis oil to biodiesel changes the fuel density (Figure 1). Studies have shown that the density increases with increasing TPO content in the biofuel mixture and decreases with increasing temperature. When using pure biodiesel and its mixtures TPO15, TPO30, and TPO60 with tire pyrolysis oil at a temperature of 20 °C, the density decreased by 6%, 4.3%, 3.2%, and 1%, respectively, compared to diesel fuel. At a temperature of 60 °C, the density of diesel fuel and tire pyrolysis oil was found to be 6.1% and 17.2% higher, respectively, than that of pure second-generation biodiesel. The increase in density with higher TPO content is primarily due to the higher molecular weight and greater aromatic compound concentration in TPO compared to HVO. Aromatic hydrocarbons tend to have denser molecular structures, which increases the overall mass per unit volume of the blend. Additionally, the presence of residual heavy compounds from the pyrolysis process contributes to increased fuel density.
Viscosity affects spray quality and fuel system performance, especially under different temperatures and blending conditions. The viscosity of the fuel affects the quality of fuel spraying and the operating conditions of the fuel supply system. When the fuel viscosity is low, the plunger pores are lubricated worse; the fuel is sprayed in smaller droplets. The kinetic energy of these droplets is lower, so they remain closer to the injector; the combustible mixture is prepared worse, and the engine may smoke. When the fuel viscosity is higher, the droplets form larger; they fly further but evaporate more slowly. The higher kinematic viscosity observed in TPO-rich blends can be attributed to the presence of long-chain hydrocarbons and polymeric residues formed during tire pyrolysis. These components increase the internal friction within the fuel, which hinders atomization during injection. As viscosity rises, droplet formation becomes less efficient, leading to larger droplets and uneven combustion, especially under low-temperature conditions.
Figure 2 shows the dependence of the kinematic viscosity of fuels of different compositions on temperature. At a temperature of 40 °C, the kinematic viscosity of biodiesel and its mixtures TPO15, TPO30, and TPO60 with tire pyrolysis oil was found to be 31.5%, 36.1%, 42.5%, and 47.2%, respectively, higher than that of conventional diesel fuels.
During data analysis, it was observed that depending on the amount of TPO added, the fuels used, and their mixtures, the kinematic viscosity decreases with increasing temperature. As can be seen in the graph, at a temperature of 60 °C, the kinematic viscosity of biodiesel and tire pyrolysis oil was found to be 26.6% and 43.5% higher, respectively, compared to diesel fuel.
The lubricity of a fuel is evaluated using a ball-on-disc wear test (HFRR), where wear is influenced by a combination of mechanisms, including boundary lubrication breakdown, adhesion, and surface fatigue, not solely abrasion. A steel ball, firmly embedded in the fuel, is rubbed against a plate at high frequency. The size of the resulting plate indicates the abrasion and is therefore a measure of the lubricity of the fuel (Robert Bosch GmbH., 2004). Figure 3 shows the average abrasion diameters of the ball using diesel fuel, biodiesel, and its blends TPO15, TPO30, TPO60, and TPO100 with tire pyrolysis oil.
When analyzing the results, it was found that the largest wear diameter of 0.345 mm was recorded using pure tire pyrolysis oil (PA100). The smallest wear diameter of 0.185 mm was obtained using diesel fuel. Meanwhile, using vegetable oil (HVO), the wear diameter was 0.207 mm. As the amount of tire pyrolysis oil in the mixture increased, the wear diameter increased. It can be seen from the graph that the average wear diameter values of the studied fuels with different compositions meet the standard requirements for diesel fuel. The observed increase in wear diameter with higher TPO concentrations may be explained by the reduction in polar lubricating compounds that are present in HVO or diesel but absent or insufficient in untreated TPO. Furthermore, despite TPO containing sulfur and some aromatic compounds, which may offer boundary lubrication, the lack of surface-active additives means insufficient film strength to prevent metal–metal contact under HFRR conditions.

3.2. Engine Performance and Emission Analysis with TPO–HVO Blends

Changes in combustion process parameters also affect engine efficiency. Figure 4 shows the BSFC engine operating at different loads.
The results presented show that the lowest BSFC values were obtained using HVO fuel. At maximum engine load (100%), the increase in BSFC was most evident with higher TPO content. It should be noted that the fuel series in the figure follows a consistent order (HVO, TPO15, TPO30, TPO60, TPO), based on increasing TPO concentration, to enhance result comparability.
Additionally, while BSFC data at 10% load are shown, they should be interpreted with caution. Internal combustion engines typically operate within 20–90% of their rated output. At very low loads, engine efficiency decreases sharply, and BSFC becomes less meaningful as an indicator of performance. Idle conditions are better assessed via other parameters, which are planned to be addressed in future studies. When the engine is powered by pure tire pyrolysis oil, the comparative effective fuel consumption increases by ~63.8%. This can be explained by the fact that tire pyrolysis oil has a lower fuel calorific value and higher density. These results are consistent with those of other scientists [11]. The increase in BSFC with higher TPO content is largely due to the lower cetane number and lower calorific value of TPO. A lower cetane number results in a longer ignition delay, which impairs the combustion phasing and reduces thermal efficiency. Additionally, more fuel mass is required to produce equivalent power due to the lower energy content per kilogram of TPO.
Brake thermal efficiency (BTE) is a key indicator of how efficiently an engine converts the chemical energy of fuel into mechanical energy. A higher BTE value reflects better combustion efficiency and lower energy losses in the engine system. Figure 5 shows the dependence of the BTE on the load when the engine is operating on fuels of different composition. The largest decrease in the effective efficiency coefficient of the engine is obtained at low engine loads. At all engine operating loads, the effective efficiency of the engine fueled with tire pyrolysis oil was lower compared to HVO fuel. The largest increase (0.3) was obtained when the engine was running on pure biofuel at full (100%) engine load. When using TPO15, TPO30, and TPO60 fuel blends, the effective efficiency decreased by 9.2%, 10.1%, and 23.9%, respectively, compared to the engine fueled with HVO fuel under the same engine operating conditions. The reduction in brake thermal efficiency (BTE) is related to inefficient combustion caused by the higher viscosity and poorer volatility of TPO. These properties lead to slower evaporation and mixing in the combustion chamber, especially at lower loads, resulting in incomplete combustion and energy losses.
Total nitrogen oxide emissions consist of nitrogen monoxide and nitrogen dioxide. This is the general name for nitrogen and oxygen compounds. Nitrogen oxides are formed during all combustion processes involving air, during side reactions with nitrogen in the air. The graph shows that total nitrogen oxide (NOX) emissions increased with increasing engine load (Figure 6). The highest NOX emission value (2182 ppm) is obtained when the engine is powered by tire pyrolysis oil. When the engine is operating at full load, the fuel mixtures TPO15, TPO30, and TPO60 increased total nitrogen oxide emissions by 5.3%, 20%, and 26.4%, respectively, compared to HVO fuel. The results obtained are largely consistent with those of other scientists who studied the influence of tire pyrolysis oil on the performance and exhaust emission indicators of an internal combustion engine. The authors suggest that NOX emissions may increase due to the higher nitrogen and sulfur content in the tire pyrolysis oil [18]. The increase in NOx emissions at high TPO concentrations and loads is explained by the longer ignition delay and higher combustion temperatures, which favor thermal NOx formation (Zeldovich mechanism). Furthermore, the nitrogen content present in TPO may contribute to fuel-bound NOx formation, adding to the overall emission levels. At 10% engine load (4.5 N·m), NOx emissions were significantly lower for all fuels. This is due to reduced in-cylinder combustion temperatures at low load, which limit the thermal NOx formation governed by the Zeldovich mechanism. The lower fuel mass and slower combustion rate also contribute to reduced peak temperatures, suppressing nitrogen oxidation reactions.
The dependence of carbon monoxide emissions on load is presented in Figure 7. As can be seen from the graph, when the engine is operating at low load (10%), carbon monoxide (CO) emissions using pure tire pyrolysis oil (TPO) were lower compared to the engine running on HVO fuel. The reduction in CO emissions at low load with TPO blends may result from the oxygenated compounds in TPO, which promote more complete oxidation of carbon species. However, at high TPO content or poor atomization conditions, incomplete combustion may reverse this trend. CO emissions were also low at 10% load. At light engine load, the air–fuel mixture tends to be lean (excess air), and combustion occurs more completely. In this condition, the oxygen available in the combustion chamber is sufficient to oxidize carbon into CO2, thus minimizing CO formation. Although TPO60 produced higher CO emissions at 100% load, its smoke opacity was lower than expected. This may be due to the presence of oxygenated compounds in TPO, which can facilitate partial oxidation of soot precursors even under incomplete combustion conditions. Moreover, the poor atomization of TPO60 at high load may shift combustion toward a less efficient but cleaner-burning mode with reduced soot nucleation, despite elevated CO production.
The smoke emission of the exhaust gases of an engine operating on second-generation biofuels and tire pyrolysis oil mixtures TPO15, TPO30, and TPO60 is presented in Figure 8. The graphs show that the engine load had the greatest impact on the smoke emission. When the engine was operating at a low load (10%), the tire pyrolysis oil generated the highest smoke emission.
At medium load (50%), when the engine was fueled with a tire pyrolysis oil TPO15 mixture, the smoke level increased by 13.3% compared to HVO fuel. At full load, the highest smoke level was obtained when the engine was fueled with tire pyrolysis oil. HVO fuel does not contain polycyclic hydrocarbons and sulfur, which could have contributed to lower smoke levels. The smoke level results obtained are consistent with the results of studies conducted by many scientists. Yaqoob et al. [11] argue that smoke level is determined by fuels with different physical and chemical properties.
TPO’s aromatic-rich composition can contribute to both soot formation and its oxidation. At low and moderate loads, the presence of inherent oxygen in TPO may assist soot oxidation. However, at higher loads, increased fuel injection and incomplete vaporization can lead to higher smoke levels due to persistent aromatic clusters and insufficient oxidation.

4. Discussion

The results obtained from this study using a single-cylinder test engine provide initial insights into the behavior of tire pyrolysis oil (TPO) and hydrotreated vegetable oil (HVO) blends, particularly with respect to engine performance, emissions, and lubricity. While the findings are limited to the specific engine and test conditions, they contribute to the broader understanding of how such blends might perform in diesel combustion systems. In the discussion section, it is appropriate to discuss the relationship of the obtained results with the works of other authors to highlight limiting factors, potential industrial significance, and opportunities for further research.
Some studies suggest that small additions of TPO may positively influence combustion, possibly due to the presence of oxygen-containing compounds or its relatively high energy density. However, it is important to emphasize that such effects are highly dependent on the specific experimental setup, including the engine type, fuel injection system, combustion chamber geometry, and test conditions (e.g., load and speed). The observed emission trends and combustion efficiency in the literature vary significantly due to differences in fuel treatment methods, engine calibration, and TPO composition. Therefore, caution should be exercised when comparing results across studies, and generalizations should be avoided without standardized testing protocols. Other researchers suggest improving TPO by adding additives. For example, dimethyl carbonate or diethyl ether are mentioned as potential additives for increasing the cetane number and reducing viscosity for TPO-based fuels. Such additives can compensate for the shortcomings of TPO, allowing its higher concentration in the mixture. Summarizing the results of previous studies, it can be stated that tire pyrolysis oil can indeed be an alternative diesel engine fuel, especially in a mixture. The recommended proportion of TPO is often mentioned in lower percentages (up to ~20%), because then there is almost no negative impact on engine performance [11]. Some authors even claim that a blend of up to 50% TPO with diesel can work, although it will not fully meet all fuel standards (e.g., will not meet cetane or distillation requirements) [1]. However, for TPO to be widely used, several technical challenges need to be solved—reducing sulfur content (e.g., by applying oxidative desulfurization methods [11]), increasing the cetane number (using additives, as suggested by Hariharan et al. [23]), as well as ensuring regulatory indicators, such as flash point, etc., which can be affected by blending large proportions of TPO. The totality of these measures would lead to TPO and its blends not only successfully generating an engine but also meeting fuel standards [24].
During engine tests, one of the most striking aspects was the changes in engine operating parameters (power, specific fuel consumption, effective efficiency) with increasing tire pyrolysis oil content in the mixture. Studies show that a small concentration of TPO (up to 15–20% by volume) in a mixture with HVO does not substantially worsen engine operating conditions and in some cases may even slightly improve the combustion process due to certain oxygen-enriched components in the TPO composition. Some other studies (e.g., [11]) also note that small TPO admixtures can improve combustion intensity and slightly reduce CO emissions. However, it is important to appreciate that the cetane number plays a very important role here: if the cetane number decreases too much (e.g., if TPO is >30%), the ignition delay may start to increase [25]. This leads to a part of the combustion taking place under steeper conditions; the combustion temperature will increase, and NOx emissions may increase at the same time [26]. Such trends have also been observed by other authors who studied the blending of TPO with diesel [18], although it is indicated that the results may vary depending on the test bench, engine design, and the variety of TPO composition.
The increase in comparative effective fuel consumption observed in this work at a high TPO content (e.g., 30–50%) is consistent with the literature, which mentions that TPO has a lower calorific value (about 38–40 MJ/kg) than traditional diesel (about 42–43 MJ/kg); therefore, in order to obtain the same power output, the engine needs more fuel mass. In addition, due to the higher viscosity, the spray quality deteriorates, and the fuel is not sufficiently homogeneously distributed in the combustion chamber, which makes part of the combustion process suboptimal, and the specific consumption increases. Such an effect is practically significant in vehicles, where overall fuel economy is important during long-term trips; an excessive amount of TPO in the mixtures can reduce competitiveness. However, if a lower TPO concentration is used (up to ~20%), the negative impact on engine efficiency is usually absent or minimal.
Another observation is that the results of this study show slightly better engine efficiency at ~50–80% load with HVO fuel compared to blends with a high TPO content. This is logical, since HVO has an extremely high cetane number (around 78–80) and a very low sulfur content. Due to its excellent combustion properties, HVO practically ensures a precise combustion process and good ignition, so even at high load, the engine operates more efficiently. A higher TPO content, bringing excess aromatic compounds, negatively affects the cetane number, so the engine faces an additional challenge to ensure even combustion.
In the area of emissions, similarities were found with the trends in the results of other authors. In this study, it was observed that
  • CO and soot (smoke) emissions often decrease with increasing TPO content (up to a certain limit). This may be because TPO contains oxygen-rich compounds and fewer saturated (paraffinic) components, which tend to form more soot during combustion. A similar effect was reported by [27], who observed that oxygen-rich compounds may improve combustion and help reduce the formation of incomplete combustion products. In addition, if TPO contains more aromatic compounds, more intense pyrolytic combustion is possible, but with good oxidation reactions, soot formation may be somewhat lower—it is important that combustion does not continue at low temperatures.
  • The changes in NOx emissions are rather ambiguous—in some regimes they increase, in others they decrease. Theoretically, aromatic compounds and a low cetane number usually delay ignition; therefore, if the delay time is increased, the peak temperature could sometimes decrease, thus reducing NOx emissions. However, if a certain part of the combustion takes place quickly and at high-temperature conditions (when a larger amount of fuel ignites immediately after the delay), NOx may increase. In addition, TPO contains sulfur and nitrogen impurities, which can promote the formation of not only NOx but also SOx. Our study showed that at high load (100% load mode), NOx increased significantly when the TPO content reached 60% and above. This observation is consistent with the findings of other publications that increased aromatic content of fuels and low cetane number can lead to more intense combustion at higher temperatures, especially at maximum load [18,28].
  • SOx emissions were not directly measured in the results section of this work, but considering the sulfur content in TPO (~0.84% according to the table), it can be assumed that under real conditions without additional TPO desulfurization, such emissions would exceed the ultra-low sulfur limits required for road transport fuels today. This indicates that in order to practically apply a large proportion of TPO, it is necessary to perform additional refining or blend with ultra-low sulfur components (for example, HVO) so that the final sulfur content in the blend meets the standards (<10 ppm in the EU EN 590 standard [29]).
Emission changes can be regulated by choosing the appropriate TPO to HVO ratio. It seems that ~10–30% TPO in a predominantly HVO base constitutes a compromise option when it is possible to maintain low smoke, relatively low CO, and NOx values, and sulfur compound levels are not yet critically high. However, it should be remembered that this range may change if technologies are applied that reduce sulfur content or increase cetane number (e.g., by adding cetane enhancer additives).
A more detailed analysis of the results shows that one of the most important problems of TPO mixtures is high viscosity and density changes. It was found that at a temperature of 40 °C, the viscosity of TPO can exceed 3.7 mm2/s (in the case of diesel, ~2.1 mm2/s). Under normal engine operating conditions, when the fuel warms up to 50–60 °C, the viscosity decreases somewhat, but TPO still remains more viscous than diesel or even HVO. In practice, this means that the fuel pump may experience a higher load, and the nozzle droplets may be larger and less dispersed in the combustion chamber. This study shows that there is a correlation between higher viscosity and increased effective fuel consumption—poorer mixture formation leads to a part of the fuel burning in a less than optimal phase, which worsens the thermal efficiency of the engine.
Increasing the density, on the other hand, can increase the mass of the injected fuel for the same volume, which sometimes improves the power performance but also increases the risk of excessive soot emission or even excessive fuel overshoot at some moments (especially at 45 N·m and a constant engine speed of 2000 min−1). This work shows that at around 15–30% TPO, this phenomenon is not drastic, but at 50–60% TPO, it may be necessary to adjust the fuel dosage or adjust the engine management program to avoid excessively rich zones of the mixture in the combustion chamber. From an industrial point of view, this means that mixtures with a higher TPO fraction must be more carefully adapted to the engine delivery system. The usual configuration of diesel injectors (e.g., common rail system) may not provide sufficient flexibility to compensate for the significantly different viscosity and evaporation characteristics. In this case, engineers may have to modify the injector nozzles or the spray pressure. An alternative solution is to use special chemically or thermally treated TPO distillates (e.g., separation of the heavy fraction), which would better match the viscosity, cetane, and other quality indicators.
Pure TPO, as shown by tribological measurements (using the HFRR method), has a larger wear diameter than standard fuels, which indicates poorer lubricity. However, it is interesting that in certain cases, sulfur compounds and aromatic resins in the TPO composition can act as lubricants. Therefore, lubricity is not unambiguous: pure TPO, with a high sulfur concentration, can simultaneously improve the lubrication of metal contacts but promote corrosion and create additional environmental problems (SOx emissions). Our study clearly showed that the highest wear diameters were achieved with 100% TPO and the lowest with pure diesel, but the differences between some blends were relatively small, remaining within the HFRR limits compatible with EN 590.
This suggests that a suitable balance of TPO and HVO (or TPO and diesel), for example 15–30% TPO, can reliably meet the lubrication requirements without additional additives, provided that the total sulphur content and certain polar components help to form a protective film. However, if the TPO content is increased to 50–60% or more, special additives may be required to ensure lubrication. The HFRR test provides a fairly accurate indication of how much additive is required to maintain a wear diameter of <460 μm. Therefore, for industrial implementation, systematic lubricity testing is recommended, especially considering that the TPO composition may vary depending on the initial composition of the tires or the pyrolysis conditions.
From an industrial perspective, the tire pyrolysis process itself can be economically viable if the processing facilities produce a sufficiently high yield of TPO and if the TPO is stably marketed as a partial or full diesel substitute. Techno-economic analyses ([17,30]) suggest that investments in pyrolysis plants would be profitable if
  • A regular supply of waste tires is ensured;
  • The resulting TPO is sold at an adequate price corresponding to its energy value;
  • The planned additional processing steps (e.g., desulfurization) do not make the fuel too expensive;
  • There is political/legal support (e.g., tax incentives for the development of waste recycling technologies).
The engine test data and HFRR lubricity data obtained in the context of this article can help industrialists assess under what conditions TPO-blended fuels would be acceptable. For example, if studies show that up to 20–30% TPO with HVO can still meet the lubricity and performance requirements of EN 590, manufacturers can offer such a blend in their product portfolio as an alternative, circular fuel for transport. This type of fuel could also be of interest in other areas, such as thermal boilers or generators, where the quality requirements are less stringent than for road transport diesel.
Many experiments, including those presented in this work, involve short-term tests, more focused on the assessment of instantaneous parameters (engine power, emissions, consumption, tribology [31,32,33]). The question remains open regarding the long-term use of such blends. Will using TPO for a longer period of time not cause more tarry deposits to appear in the fuel system? Will sulfur compounds promote corrosion in high-pressure pumps and injectors? Will the fractions that do not meet the requirements, which may form during combustion, eventually become volatile deposits on the piston or valves? There is a lack of detailed data in the scientific literature on this [1,34]. Therefore, it is necessary to conduct longer-term tests, perhaps simulating real vehicle operation for thousands of hours, to determine the extent of deposit formation and wear of engine parts. Only after accumulating such practical operational experience will it be possible to confidently confirm that the use of TPO mixtures does not pose a threat to engine reliability.
Another important issue is the compatibility of intermediates. Some fuels with high aromatic content can swell or destroy rubber gaskets and O-rings. In reality, engines designed for diesel may have other elastomeric parts that are not intended for contact with high aromatic content. In order to commercialize fuels with TPO, interaction tests with such compounds may have to be performed. This aspect may be very relevant in future research.
In order for TPO to become competitive in exchange for HVO or other biofuels, it is necessary to address two main limiting factors: too low a cetane number and too high a sulfur content.
Cetane enhancement:
  • Additives (e.g., 2-ethylhexylnitrate (2-EHN), di-tert-butyl peroxide, etc.)—these compounds are widely used in industry as cetane enhancers. The literature indicates that adding a few percent by volume of 2-EHN can increase the cetane number by as much as 10 units [35,36].
  • Distillation modification: If the TPO fraction had fewer heavy aromatics or resinous fractions, then the increased ratio of aliphatic components could improve ignition properties [37,38].
  • Blending with ultra-high cetane fuel components, e.g., HVO. HVO itself has a cetane number of ~78–80, so it could compensate for the low TPO indicator, as long as the total cetane number of the mixture does not fall below ~50.
Sulfur reduction:
  • Catalytic hydrodesulfurization (HDS) is a traditional oil industry method that requires high temperatures, hydrogen, and a catalyst (e.g., Co-Mo, Ni-Mo) [39,40]. Such equipment can be more expensive, but it allows for ultra-low sulfur reduction.
  • Oxidative desulfurization (ODS) is a more innovative method suitable for lean streams. With the help of special oxidants (e.g., hydrogen peroxide) and a catalyst, sulfur compounds are converted into sulfones, which are washed out during liquid phase extraction [41,42].
  • Biological desulfurization is still mainly a research topic [43], but in the future it could allow for sulfur reduction with lower energy costs.
Additionally, the long-term stability of TPO-based fuel blends remains an open question. Future studies should evaluate how storage time affects the physicochemical properties of these mixtures, including potential phase separation (stratification), oxidation, or degradation of reactive compounds. These aspects are critical for ensuring consistent fuel quality, especially for industrial-scale implementation where storage and transportation are inevitable.
Thus, by applying at least one or more of these technologies, it would be possible to obtain TPO with properties closer to diesel. Although this increases the cost of the product, it could open up wider opportunities for such fuels to enter the market and comply with legal directives on emissions.

5. Conclusions

  • A small concentration of TPO in mixtures with HVO (up to ~20%) does not significantly affect the effective fuel consumption or power, but exceeding this limit significantly increases the ignition delay and increases the overall fuel consumption.
  • Studies have shown that at higher temperatures (40–60 °C), the viscosity decreases but remains higher than that of diesel or HVO. This makes fuel spraying more difficult and may contribute to higher fuel consumption.
  • In some load modes, NOx decreased due to the longer ignition delay, but at a high TPO fraction and maximum load, NOx increased significantly. This effect is determined not only by the decrease in cetane number but also by the sulfur and nitrogen impurities in the TPO composition.
  • Due to the oxygen-enriched compounds and different balance of aromatic compounds in TPO, combustion becomes more intense, and some soot is oxidized more efficiently. This allowed the reduction of smoke, but the dependence on engine load remains high.
  • Lubricity (HFRR method) is best when using conventional diesel or HVO, but wear diameter indicators obtained in blends with a limited amount of TPO (up to ~30%) still meet the requirements of EN 590. Employing a higher proportion of TPO may necessitate the incorporation of additional additives to improve lubricity indicators.
  • The TPO exhibited a sulfur content of approximately 0.84% during the trial, significantly surpassing the ultra-low sulfur threshold of less than 10 ppm. Desulfurization techniques hold significant importance in industrial applications, as elevated sulfur concentrations lead to the production of acid gas emissions and can promote corrosion within fuel systems.
  • The obtained short-term results confirm the potential for using TPO blends with HVO, but longer-term tests are necessary to assess the potential for deposit accumulation, changes in oil properties, and loads on gaskets or injectors.
  • In order for TPO to become a sustainable, industrially acceptable alternative, efficient tire pyrolysis, refining (e.g., desulfurization, cetane number enhancement), and properly balanced blends with second-generation biofuels are required to ensure sufficient engine power, compliance with emission standards, and reliable fuel system operation.

Author Contributions

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

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Temperature dependence of fuel density of different compositions.
Figure 1. Temperature dependence of fuel density of different compositions.
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Figure 2. Dependence of kinematic viscosity of fuels of different composition on temperature.
Figure 2. Dependence of kinematic viscosity of fuels of different composition on temperature.
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Figure 3. Average wear diameters of balls using different fuel compositions.
Figure 3. Average wear diameters of balls using different fuel compositions.
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Figure 4. BSFC dependence of engine load when the engine was running on different fuels and their mixtures. (The fuel series is arranged by increasing TPO content (HVO → TPO). BSFC at 10% engine load is presented for completeness but should be interpreted cautiously, as real-world engines typically operate above 20% load.)
Figure 4. BSFC dependence of engine load when the engine was running on different fuels and their mixtures. (The fuel series is arranged by increasing TPO content (HVO → TPO). BSFC at 10% engine load is presented for completeness but should be interpreted cautiously, as real-world engines typically operate above 20% load.)
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Figure 5. BTE dependence of engine load when the engine was running on different fuels and their mixtures. (The fuel series is ordered by increasing TPO content for consistency. As with BSFC, BTE values at very low loads (10%) should be interpreted with caution, as such conditions fall outside the normal operating range for most internal combustion engines.)
Figure 5. BTE dependence of engine load when the engine was running on different fuels and their mixtures. (The fuel series is ordered by increasing TPO content for consistency. As with BSFC, BTE values at very low loads (10%) should be interpreted with caution, as such conditions fall outside the normal operating range for most internal combustion engines.)
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Figure 6. NOx emissions dependence of engine load when the engine was running on different fuels and their mixtures.
Figure 6. NOx emissions dependence of engine load when the engine was running on different fuels and their mixtures.
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Figure 7. CO emissions dependence of engine load when the engine was running on different fuels and their mixtures.
Figure 7. CO emissions dependence of engine load when the engine was running on different fuels and their mixtures.
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Figure 8. Smoke dependence of engine load when the engine was running on different fuels and their mixtures.
Figure 8. Smoke dependence of engine load when the engine was running on different fuels and their mixtures.
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Table 1. Properties of different fuels. (The lubricity value (HFRR, wsd 1.4 at 60 °C) listed under EN 590 refers to the maximum acceptable wear scar diameter, not measured values.)
Table 1. Properties of different fuels. (The lubricity value (HFRR, wsd 1.4 at 60 °C) listed under EN 590 refers to the maximum acceptable wear scar diameter, not measured values.)
PropertiesEvaluation MethodDDHVOTPOEN590
Density at 15 °C, kg/m3EN ISO 12185:1999832.7779.8910800–845
Kinematic viscosity at 40 °C, mm2/sEN ISO 3104+AC:20002.132.923.771.5–4
Lubricity properties adjusted for diameter wear (HFRR), (wsd 1.4) at 60 °C, µmEN ISO 12156-1---Max: 460
Cetane numberEN ISO 5165:199951.478.939Min: 51
Oxygen content, max wt%-001.76
Carbon-to-hydrogen mass ratio (C/H)-6.625.58.26
Table 2. Fuels and mixtures of fuels used for research.
Table 2. Fuels and mixtures of fuels used for research.
Testing FuelsHVO Amount Vol %TPO Amount Vol %
HVO1000
TPO158515
TPO307030
TPO604060
TPO0100
Table 3. Properties of tire pyrolysis oil, HVO fuel.
Table 3. Properties of tire pyrolysis oil, HVO fuel.
PropertiesMeasurement MethodTPOHVO
Flash point in open cup (FP), °CEN ISO 2719:20004379.5
Stoichiometric air–fuel ratio, kg/kg-13.4615.1
Normal calorific value, MJ/kgEN ISO 8217:201240.4943
Cetane numberEN ISO 5165:199939.9478.9
Carbon (%) 86.6884.6
Hydrogen (%) 10.4915.39
Oxygen (%) 1.29−
Nitrogen (%) 0.48−
Sulfur (%) 0.84−
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Mickevičius, T.; Dudziak, A.; Matijošius, J.; Rimkus, A. Evaluation of Tire Pyrolysis Oil–HVO Blends as Alternative Diesel Fuels: Lubricity, Engine Performance, and Emission Impacts. Energies 2025, 18, 4389. https://doi.org/10.3390/en18164389

AMA Style

Mickevičius T, Dudziak A, Matijošius J, Rimkus A. Evaluation of Tire Pyrolysis Oil–HVO Blends as Alternative Diesel Fuels: Lubricity, Engine Performance, and Emission Impacts. Energies. 2025; 18(16):4389. https://doi.org/10.3390/en18164389

Chicago/Turabian Style

Mickevičius, Tomas, Agnieszka Dudziak, Jonas Matijošius, and Alfredas Rimkus. 2025. "Evaluation of Tire Pyrolysis Oil–HVO Blends as Alternative Diesel Fuels: Lubricity, Engine Performance, and Emission Impacts" Energies 18, no. 16: 4389. https://doi.org/10.3390/en18164389

APA Style

Mickevičius, T., Dudziak, A., Matijošius, J., & Rimkus, A. (2025). Evaluation of Tire Pyrolysis Oil–HVO Blends as Alternative Diesel Fuels: Lubricity, Engine Performance, and Emission Impacts. Energies, 18(16), 4389. https://doi.org/10.3390/en18164389

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