Assessment of Engine Performance and Emissions with Eucalyptus Oil and Diesel Blends

Abstract

This research evaluates the feasibility of using eucalyptus oil blended with conventional diesel fuel in diesel engines. Eucalyptus globulus is one of the main tree species cultivated for paper pulp in western European countries such as Portugal, and eucalyptus oil is one of the byproducts that so far has not been sufficiently evaluated as a biofuel. This study assesses the impact of using this additive on engine performance parameters and emissions as a means to contribute to reducing fossil fuel consumption and pollutant and greenhouse gas (GHG) emissions. The analysis revealed that the addition of eucalyptus oil had a positive effect on torque, a critical performance parameter, with biofuel blends showing consistent torque increases at lower engine speeds. However, torque tended to decrease towards the higher range of engine speed for eucalyptus oil–diesel blends. Several blends showed lower brake specific fuel consumption compared to regular diesel at high engine loads and low engine speeds. Brake thermal efficiency did not vary substantially at lower engine speeds and loads but decreased at higher speeds and loads. Pollutant emissions, particularly unburned hydrocarbons and nitrogen oxides, were influenced by fuel composition, with biofuel blends showing both increases and decreases compared to diesel. It is noteworthy that eucalyptus oil blends exhibited up to a 60% reduction in smoke opacity under specific operating conditions at low speed and high load for 10% incorporation (10EU90D), suggesting that in addition to the already positive effects of cutting down fossil CO₂ emissions in proportion to the substitution of fossil diesel with nearly carbon-neutral eucalyptus oil, more environmental benefits may be expected from the incorporation of this product. Although the present economic viability of using eucalyptus oil as a biofuel is still not guaranteed, the present study seems to reinforce its technical viability. Future prospects for the improvement of oil yield through biotechnology, the economic interest of this product for several countries, and the updating and upscaling industrial processes may allow the viability of this biofuel to remain a possibility in the future

1. Introduction

In an era characterized by an escalating concern for the environment and the need for sustainable practices, the utilization of biofuels in internal combustion engines emerges as a promising solution for the reduction in greenhouse gas emissions and the decrease in reliance on fossil fuels. Derived from organic sources such as plants or agricultural byproducts, biofuels present themselves as a viable and ecologically friendly alternative to conventional fuels when sustainably manufactured. Moreover, these biofuels can often be used as substitutes for petrol or diesel within existing engines, thereby eliminating the necessity for significant alterations in infrastructure or technology as in the case of fuels such as hydrogen [1]. They contribute towards the mitigation of carbon emissions as the carbon in their molecules is captured by the plant from the atmosphere, and thus they are mostly carbon-neutral [2]. Additionally, biofuels also play a pivotal role in fostering local economic development and reducing reliance on oil imports. However, it is important to acknowledge the existence of challenges associated with the production and utilization of biofuels, including issues such as competition with food crops, excessive exploitation of natural resources, and the potential impact on biodiversity. Consequently, the promotion of sustainable practices in the production and utilization of biofuels becomes imperative in order to maximize the associated benefits while simultaneously minimizing any detrimental effects on the environment and society [3].

In the realm of diminishing carbon emissions and augmenting energy sustainability, the production of biodiesel in Europe is gaining increasing significance and allure [4]. Biodiesel, which originates from renewable origins such as vegetable oils or animal fats, presents a promising resolution to curtail the carbon footprint of the transportation sector. Throughout the years, Europe has fortified its position as a frontrunner in the advocacy of renewable energies, and biodiesel production has emerged as a pivotal pillar in this transition towards a low-emissions economy. Nevertheless, the process of creating and incorporating biodiesel into the European energy market is not bereft of challenges and impediments, thus the merit of finding alternative solutions for sustainable diesel fuel substitutes [5].

Eucalyptus oil, derived from the foliage and branches of the eucalyptus tree, possesses not only aromatic qualities, but also serves as a valuable asset with numerous applications, including its use as a source of fuel [6]. This fluid exhibits properties that render it highly suitable for deployment in various energy-related contexts. In addition to its invigorating and revitalizing scent, eucalyptus oil encompasses compounds that grant it a considerable potential in the realm of alternative energy sources. Through appropriate refinement and treatment procedures, this oil can emerge as an appealing choice for energy production, constituting a sustainable and ecologically friendly substitute for conventional fuels.

Eucalyptus oil consists predominantly of a diverse array of organic compounds, which confer upon it properties that render it suitable for utilization as a source of energy. The primary constituent of eucalyptus oil comprises 1,8-cineol, which serves as the primary chemical compound in eucalyptus oil. This compound represents between 70% and 90% of the overall composition and possesses a considerable combustion potential. Additionally, alpha-pinene and beta-pinene play a pivotal role in bestowing upon eucalyptus oil its distinctive fragrance and augmenting its combustibility. Another prevalent compound encountered in eucalyptus oil is limonene, an aromatic hydrocarbon that can serve as a fuel source. Furthermore, eucalyptus oil may contain other terpenes and aromatic compounds, the specific composition of which may vary contingent upon the species of eucalyptus and the techniques employed for extraction and refinement [7,8]. Using eucalyptus oil as a fuel may yield a reduced impact on carbon dioxide emissions in comparison with petroleum, as it originates from plant resources and is biodegradable and renewable. When combusted, it releases carbon dioxide into the atmosphere, but this may be considered a neutral emission as it is compensated for by the plants’ absorption of carbon during their growth [9]. Thus, the utilization of eucalyptus oil as a fuel can be deemed carbon-neutral by adhering to sustainable practices in its harvesting and cultivation. For instance, if eucalyptus plants are cultivated on previously degraded or deforested land and implemented with soil regeneration and resource management techniques, then they can aid in the sequestration and retention of carbon in both the soil and plants. Consequently, this offsets the carbon emissions generated during the production and usage of eucalyptus oil as a fuel. In addition to its significant economic contribution to Portugal, eucalyptus is a fast-growing species, making it highly effective at capturing carbon dioxide and combating the greenhouse effect. The introduction of this species to Portugal occurred around 1830, but the expansion of eucalyptus plantations saw substantial growth in the second half of the 20th century, alongside the development of the country’s pulp and paper industry. As an example, the leading paper manufacturers in Portugal, who is also a major player in the global pulp, oil [10], and paper industry from eucalyptus tree estimates that its power plants avoid emitting over 460 thousand tons of CO₂, according to the national assessment [11]. The company further reported in 2021 that the eucalyptus forests it manages can sequester up to 5.9 Mton of carbon dioxide annually. This does not seem to be a negligible value in the scope of Portugal’s environmental goals. To assess the technical feasibility of using eucalyptus oil as a partial substitute (additive) of commercial diesel fuel thus seems to be an important endeavor, even if the economic viability is still not guaranteed.

Although the utilization of eucalyptus oil in numerous domains has been observed, its exploration and application as a renewable energy source has not yet attained extensive prevalence. Relatively few research endeavors have been undertaken to investigate the integration of this fuel into diesel engines [12,13,14,15]. The research conducted by [16] found that the fuel consumption of two samples of pure eucalyptus oil exhibited a decrease of 8.18% and 4.05%, respectively, compared to diesel fuel when subjected to maximum load conditions. The study also found similar patterns in brake fuel consumption, as documented in the referenced work [6]. The injection strategy and compression ratio in engine performance has been emphasized in [12]. The engine power, cylinder pressure, and torque hold significant importance in relation to engine performance, and their values depend upon the chosen methodology. Typically, when utilizing combinations of eucalyptus oil and diesel or solely pure essential oil (eucalyptus oil), there is a tendency to enhance the engine’s developed torque and power [17].

In a diesel engine with an unchanged compression ratio, optimal performance was noted when utilizing B100 (undiluted biodiesel) under full load conditions. This resulted in a commendable brake thermal efficiency (referred to as BTE) of 33.6%, which corresponds to a specific fuel consumption (SFC) of 0.31 kg/kWh [18].

The eucalyptus oil fuel demonstrated a noteworthy reduction in unburned hydrocarbons and particulate emissions in comparison to both waste plastic oil (WPO) and diesel fuel [19]. Similar findings have been documented in another paper [20]. The gaseous discharges of eucalyptus-derived biofuel have successfully observed the prevailing guidelines for exhaust emissions, evidencing reductions in the levels of NOx, hydrocarbons, and carbon monoxide.

In previous studies, the authors have investigated biofuels that seem to display a substantial potential for the partial substitution of gasoline [21,22] and diesel fuels [23]. The present investigation conducted a sequence of experiments involving combinations of eucalyptus oil and diesel fuel in varying ratios used as fuel of a 1.6 L direct-injection, supercharged, four-cylinder, common-rail diesel engine. To reduce the complexity of the analysis and allow for some insight into the impact of fuel blends, some parameters that normally vary in engine operation were prevented from changing. This was the case for the EGR level (EGR system was deactivated) and the inlet pressure (it was kept at 0.5 bar throughout the testing process).

The present study focused on evaluating the performance characteristics and pollutant emissions of biodiesel blends containing eucalyptus oil, an alternative biofuel with potential environmental and economic benefits, especially in western European countries like Portugal. Biofuels derived from renewable sources such as eucalyptus oil have gained considerable attention due to their potential to reduce greenhouse gas emissions and dependence on fossil fuels. In this context, the performance evaluation includes torque characteristics at various engine speeds and loads, aiming to elucidate how different blend compositions, particularly the incorporation of eucalyptus oil, influence engine efficiency and power output. Additionally, the study investigates pollutant emissions such as particulate matter (PM), nitrogen oxides (NOx), and carbon monoxide (CO₂), crucial parameters for assessing the environmental impact of biofuel utilization. By examining both performance metrics and emission profiles, this research contributes to advancing the understanding of eucalyptus oil biodiesel blends as viable alternatives in the pursuit of sustainable energy solutions. The main novelties of this work seem to be the testing of a seldomly tested biofuel, eucalyptus oil, which has a high economic value in Portugal and other western Europe countries. More specifically still, these tests were performed by comparing the results of several blends of this biofuel for the operating conditions that a motor vehicle would experience under typical road and highway conditions instead of full (wide-open throttle) or fixed partial load conditions, as is typical of many analyses. This test method is particularly relevant because it more faithfully replicates the conditions of daily vehicle use. Additionally, most of the existing research on this biofuel was conducted with outdated, low-pressure mechanical injection systems, instead of newer, high-pressure common-rail systems. Mechanical injection systems do not require high pressures and are less expensive if deposits or other technical problems occur. However, these systems do not nearly reflect the actual performance and behavior of vehicles equipped with modern injection systems. Therefore, testing biofuels under these conditions provides more complete and accurate insight into their efficiency and viability in everyday use.

Several blends of eucalyptus oil and regular diesel were tested. The nomenclature used for the mixtures was xEUyD for an x% of eucalyptus oil and y% of diesel fuel. The mixtures tested were 5EU95D, 10EU90D, 15EU85D, 20EU80D, and 30EU70D. The performance of these blends was evaluated in four different combinations of engine speed and load, which are representative of various driving scenarios. These combinations included speeds of 1700 and 2250 rpm, which in light vehicles correspond roughly to speeds of approximately 90 km/h and 120 km/h, respectively. Two different loads were also considered to specify different road (uphill and downhill) conditions. The performance characteristics, such as torque, consumption, and efficiency, as well as the emissions of pollutants were carefully analyzed and compared.

2. Materials and Methods

In this research project, the NOVA University of Lisbon supplied the eucalyptus oil, which was subsequently mixed with diesel fuel in order to evaluate the impact of this additive on the combustion properties, performance of the engine, and exhaust emissions. Elaboration on the customary procedure for preparing this specific fuel, along with its primary attributes, is explained in the next section.

2.1. Biofuel Preparation

Figure 1 depicts one of the methods for extracting biofuels based on essential oils, which in the present case involves distilling eucalyptus leaves with steam.

The standard procedure displayed in Figure 1 is used for temperature-sensitive materials that are soluble in water and can decompose [24]. Additional information regarding the process of distilling eucalyptus oil with steam is available in other publications [25,26]. The authors report that this method is inexpensive and effective when compared to other methods. Steam distillation represents a specialized distillation technique utilized for the separation of temperature-sensitive substances such as oils, resins, or hydrocarbons, which exhibit insolubility in water and may undergo decomposition at their respective boiling points. The underlying principle of steam distillation lies in its capability to facilitate the distillation of a compound or a mixture of compounds at a significantly lower temperature compared to the boiling point of the individual constituent. Essential oils often encompass components with boiling points exceeding 200 °C or even higher temperatures. Nonetheless, in the presence of steam or boiling water, these constituents undergo volatilization at temperatures close to 100 °C at atmospheric pressure. The fresh or dried plant material is introduced into a special chamber of the still, and steam is passed through it under pressure, so as to soften the cells and allow the release of the essential oil in the form of vapors. The steam temperature must be sufficiently high to vaporize the essential oils, but not so high as to cause damage to the tissues or the burning of the oils. As the essential oils are released, they evaporate as very fine droplets, traveling along with the steam molecules through a tube to the condensation chamber of the still. Once the steam cools down, it reverts back to water, while the essential oils accumulate as a film on the surface of the water. To separate the essential oils from the distilled water, the film is then removed or decanted. The remaining water, a byproduct of the distillation process, is known as floral water.

2.2. Biofuel Properties

The use of eucalyptus biodiesel and its blends as fuel in diesel engines is of increasing interest due to their numerous benefits. Firstly, these fuels have physic-chemical properties comparable to traditional diesel, which facilitates their use in existing infrastructure without requiring significant modifications to engines or refueling facilities. This makes the transition to eucalyptus biodiesel a feasible and convenient option for the transportation industry and other sectors utilizing diesel engines. Furthermore, one of the main advantages of eucalyptus biodiesel is its potential to improve air quality by reducing pollutant emissions. Studies indicate that the use of this fuel can contribute to the reduction in carbon monoxide and particulate matter emissions compared to conventional diesel. Thus, eucalyptus biodiesel and its blends represent an environmentally friendly and sustainable solution for the transportation sector, contributing to environmental protection and air quality improvement [27].

Table 1. Typical properties of diesel and eucalyptus oil, adapted from [23,28].

Properties	Diesel	Eucalyptus Oil (Sample A)	Eucalyptus Oil (Sample B)
Formula	C12H23	C10H18O	C10H18O
Density (kg/m3)	830	913	930
Boiling point (°C)	180–340	175	178
Viscosity (cSt) @40 °C	2.5	3.2	3
Latent heat of vaporization (kJ/kg)	230	305	318
Lower heating value (kJ/kg)	42,500	43,270	44,100
Flash point (°C)	74	93	98
Auto ignition temperature (°C)	254–258	300–330	300–330
Cetane number	40–55	52	55

presents the properties of two samples of eucalyptus oil and diesel fuel. It is observed that the values for eucalyptus oil are somewhat different from those of diesel. Despite this fact, eucalyptus oil has the potential to be well-blended with diesel to create a new fuel mixture. Notably, it often displays a higher calorific value than diesel, which is good.

2.3. Experimental Setup

The engine utilized in the experiments (Figure 2) was a 1.6 L PSA HDI four-cylinder, turbocharged diesel engine with an intercooler located in the intake manifold. The engine is water-cooled and four-stroke. Modifications were solely made to the turbocharger pressure (set always at 0.5 bar absolute pressure) and the EGR, which was disabled, to facilitate the analysis of engine behavior. These alterations were implemented in order to prevent an excessive number of variables from simultaneously fluctuating, thus impeding a comprehensive analysis of the impact of fuel mixtures on engine behavior. Specifically, the turbocharger pressure was set at a constant value across the entire range of speeds to ensure a uniform turbocharging pressure for all tests, irrespective of the speeds and loads examined. While these modifications did somewhat restrict the scope of the obtained results, they facilitated an impartial evaluation of the effect of eucalyptus oil mixing under varying test conditions. The original engine control unit (ECU) was employed but underwent reprogramming to rectify any errors associated with operating the engine outside of the vehicle and to accommodate the aforementioned engine modifications. A rotary encoder, capable of measuring both 1 and 2500 pulses per rotation, was installed in the electromechanical brake shaft to monitor its rotational speed. The brake controller adjusted the braking torque in order to maintain the engine speed within a range of ±5 rpm from the reference value. The engine was equipped with the standard high-pressure fuel pump and a common-rail supply system. The injection command was executed electronically through the electronic control unit (ECU). To measure the power generated, the engine was coupled to an electromagnetic brake (Telma AD6-55, from Telma Company, Paris, France) using eddy currents. An S-type 500 kg Zemic B3G load cell (Telma Company, Paris, France) with a combined full-scale inaccuracy of ±0.02% was used to measure the torque. This resulted in an engine torque inaccuracy of no more than ±0.1 N·m when taking the load cell installation and the 3.86:1 transmission ratio into account. In terms of power, the torque was multiplied by the angular speed. For the worst possible combination of torque and speed, there was a 0.93% power calculation inaccuracy. A particulate filter was not installed, enabling a direct assessment of particulate emissions via opacity measurements. Moreover, no post-treatment systems were installed to facilitate their precise evaluation. Temperature monitoring of the coolant flow, intake manifold, lubricant, and exhaust was conducted utilizing type-K thermocouples connected to a National Instruments (NI 9214) data acquisition system (National Instruments, Austin, TX, USA), which was interfaced with a graphical interface for synchronized monitoring and recording of all parameters. The engine load setting was also monitored within the same interface using the data acquisition board provided by National Instruments.

The concentrations of pollutant emissions from the exhaust gas were measured using an AVL DIGAS 4000 Light analyzer (AVL List GmbH, Graz, Austria), which measures HC, CO₂, CO, and NOx emissions. An AVL DISMOKE 4000 instrument (AVL List GmbH, Graz, Austria) was used to measure the smoke. The reference [29] contains the device’s technical specs as well as the measurement range.

Data on the internal combustion engine used are presented in

Table 2. Engine specifications.

Engine	In-Line 4 Cylinder, PSA HDI
Fuel injection	diesel common rail
Displacement	1560 cm3
Compression ratio	18:1
Power	75 HP/56 kW @ 4000 rpm
Torque	170 N∙m @ 1700 rpm

.

A Kern PCB3500-2 precision weighing scale (KERN & SOHN GmbH, Balinger, Germany) with a resolution of ±0.01 g was employed to evaluate fuel consumption over time. The scale was integrated with the data acquisition system to record experimental data continuously and in real time. Utilizing least-squares analysis applied to the stable segment of the curve, the gradient of the mass-versus-time curve was determined, facilitating the calculation of fuel consumption. The number of samples utilized for deriving the consumption value (slope of the curve) approached or even exceeded one thousand samples during measurements spanning several minutes. Notably, fuel consumption exhibited minimal oscillation, as evidenced in the data. In the majority of cases, the coefficient of determination (R²) for the fit exceeded 0.999.

At least three measurements were made for each test condition. Records were made of the results only when the engine’s operation was stabilized, after the temperatures of the coolant and lubricant, along with the quantities of pollutants, were stabilized. The average values of the various test repetitions are represented in the results. Repetition differences were typically rather small. For example, errors in fuel consumption were typically within 1% of the mean, and errors in pollutant emissions were typically less than 3%.

2.4. Test Parameters

The test procedure consisted of evaluating each fuel mixture (eucalyptus oil and diesel) at two different engine loads and two different speeds. For the base fuel, two fixed torque values of 58 N∙m and 87 N∙m at the same speeds were imposed. These torque values correspond to the engine loads seen in

Table 3. Test conditions.

Eucalyptus oil incorporations tested [%] Example: 5EU95D = 5% eucalyptus oil + 95% diesel (mixed by volume)	5%, 10%, 15%, 20%, 30%
Engine speed [rpm] (constant speed)	1700		2250
Engine load (for all fuels) [%]	27.1	34.0	31.6	37.3
Equivalent driving conditions (constant)	Road 90 km/h		Highway 120 km/h

for each torque/speed combination. These engine loads were chosen to simulate a light car traveling at a constant speed of 90 to 120 km/h under two different road conditions.

For these conditions, the eucalyptus oil and diesel blends were subjected to the same engine loads (also 58 and 87 N∙m) for all, with slightly different results depending on fuel performance. This approach of using a constant torque instead of a constant throttle position for each test was already used in previous studies by this group and seems especially useful in performing comparisons. Actually, it makes more sense to compare conditions that yield the same intended vehicle behavior than to compare conditions with the same throttle position that do not necessarily yield the same output [30].

3. Results and Discussions

This section analyses the results of experimental tests, focusing on the performance characteristics and pollutant emissions of the engine fueled with diesel as the reference fuel, compared to blends of diesel and eucalyptus essential oil, under the same engine load and operating (rpm) parameters.

3.1. Engine Performances Parameters

3.1.1. Engine Torque

In Figure 3, engine torque is highlighted based on the measurements performed. The results illustrate a significant correlation between torque variation and the composition of fuel blends. At low engine speeds (1700 rpm), a general increasing trend of torque with the increase in biofuel incorporation was observed for both load levels. Noteworthy is the fact that the blend with the highest incorporation of eucalyptus oil (30%, 30EU70D) exhibited the most pronounced performance improvement relative to the reference fuel in both engine loads for the 1700 rpm speed. Specifically, this improvement was 6.5% and 8.8% for the low engine load (27.1%) and high engine load (34.0%) settings, respectively.

For the higher speed of 2250 rpm, typical of highway driving, the influence of eucalyptus oil incorporation on torque was not always positive, peaking at an incorporation of only 5% (5EU95D) for both engine loads. This improvement was 1.4% and 2.5% above the reference fuel for the lower and higher engine load settings, respectively. For the low load setting, 5% was the only incorporation to yield an actual improvement in torque, but for the high load setting, incorporations below 20% were still beneficial to torque. For this speed setting (2250 rpm), the most significant decrease in torque compared to the reference fuel was recorded for the highest incorporation (30EU70D), with a 5.5% and 6.6% drop in torque for the lower and higher engine load settings, respectively. These findings underscore the complex interplay between biofuel content and engine dynamics, suggesting that optimal performance gains can vary significantly depending on engine speed and load. Future research could explore further refinements in blend ratios and engine calibration to maximize the benefits of eucalyptus oil in enhancing engine torque and efficiency across a broader range of operating conditions. Similar trends of torque increase are also reported in [17].

3.1.2. Brake Specific Fuel Consumption (BSFC)

The brake specific fuel consumption can be observed in Figure 4. For most engine speeds and loads, there was an increase in fuel consumption whenever incorporating eucalyptus oil in any percentage. A notable exception occurred for the low speed (1700 rpm) and high engine load (34.0%). In this scenario, the fuel consumption experienced a significant decrease for all eucalyptus oil blends, with the strongest decrease being 7.3% relative to the reference fuel for the 15EU85D blend. This observation suggests the potential for optimizing fuel consumption under specific operating conditions through the appropriate utilization of specific fuel blend types. Having said that, it should be stated that when analyzing blends of fuels with dissimilar heating values, a lower BSFC does not necessarily mean a higher energy conversion efficiency. For that, brake thermal efficiency is a better factor. Therefore, future research efforts could focus on refining fuel blend formulations to maximize both fuel efficiency and overall energy conversion efficiency across a broader range of operating conditions. Nevertheless, in [6,16], the authors report a reduction in brake specific fuel consumption by 8% and 4%, respectively, even under maximum load conditions. The lower fuel consumption observed in the mentioned study can also be attributed to the higher calorific value of pure eucalyptus oil. In contrast, in the present research, only mixtures of eucalyptus oil and diesel were used.

3.1.3. Brake Thermal Efficiency (BTE)

The brake thermal efficiency is a fundamental measure of energy utilization in an engine, reflecting its ability to convert the chemical energy of the fuel into useful mechanical energy. This efficiency is defined as the ratio of the actual mechanical power produced by the engine to the total theoretical thermal power released by the combustion of the fuel. In other words, it represents the percentage of the available energy in the fuel that is actually used to generate power at the engine crankshaft, while the remaining energy is lost as heat or in other internal processes of the engine.

Brake thermal efficiency can be observed in Figure 5. For the lower engine speed (1700 rpm), only slight oscillations between fuel blends are observed for the lower engine load (27.1%). Thus, the higher torque caused by eucalyptus oil incorporation noticed in Figure 3 for these tests does not seem to have been caused by a higher conversion efficiency but should rather be explained mainly by the higher heating value of the eucalyptus oil. However, a higher brake thermal efficiency was observed for all incorporations of eucalyptus oil for the higher load value and lower speed. The highest value in this respect was recorded for the blend with 15% eucalyptus oil and 85% diesel, which was 7.2% more efficient than the reference fuel.

For the higher engine speed setting (2250 rpm), regardless of the load, the brake thermal efficiency of eucalyptus oil blends was similar or lower than the reference fuel, decreasing by 5.7% for the lower engine load setting (31.6%) and 8.3% for the higher engine load (37.3%). This trend is consistent with the higher fuel consumption and lower engine torque of eucalyptus oil blends noted above for these conditions. Nevertheless, the BTE was not significantly affected by eucalyptus oil incorporations up to 15%. Blends of eucalyptus oil have different combustion properties compared to diesel. These properties may include burning rate, flame stability, and behavior during injection and ignition. These differences can influence the combustion process in the engine and can affect brake thermal efficiency. To ensure complete and efficient combustion, it is important that the stoichiometric ratio of the fuel–air mixture is optimized. Blends of eucalyptus oil may require additional adjustments to the air–fuel ratio or other operating parameters to achieve efficient combustion, which can negatively impact thermal efficiency. The fuel injection system used in this research is calibrated to maximize combustion efficiency with conventional fuels such as diesel. The use of eucalyptus oil blends requires slight adjustments in injection strategy to ensure efficient and complete combustion, thereby affecting the thermal efficiency of the engine. However, brake thermal efficiency is inversely proportional to brake specific fuel consumption (BSFC). As BSFC increased, efficiency decreased in this study. Conversely, in [18], increased braking efficiency was reported when a pure biofuel was used.

3.2. Emissions

3.2.1. Unburned Hydrocarbons

Unburned hydrocarbons (HCs) mostly come from fuel that has not burned or has only burned partially. Reasons for this include defective atomization and/or combustion, an incorrect air–fuel ratio, droplets formed on the injector surface that fail to burn, fuel that does not burn due to flame quenching near walls, because it is trapped in engine crevices or dissolved into lubricant films, among other reasons [31,32]. It has a negative health impact and global warming potential. The concentration of HCs, in ppm, are shown in Figure 6 for all the conditions tested. It may be seen that for the lower engine speed (1700 rpm), HC emissions increased with any incorporation of eucalyptus oil, for both low and high imposed loads. In these two scenarios, the 20EU80D blend showed significant increases of 12.3% and 23.3%, respectively, compared to diesel. In contrast, for the higher engine speed (2250 rpm), the opposite occurred: there was a reduction in HC emissions for all incorporations of eucalyptus oil. The higher emissions of hydrocarbons when using blends of eucalyptus oil compared to conventional diesel can be attributed to differences in chemical composition, combustion properties, combustion efficiency, and fuel-refining standards, all of which contribute to how hydrocarbons are formed and emitted in the exhaust.

3.2.2. Carbon Dioxide

In the internal combustion engine, the process of burning liquid fuels is inevitably accompanied by the release of carbon dioxide (CO₂). It may not be properly called a pollutant, as it is only perilous to health due to asphyxiation (lack of oxygen) at high concentrations. However, its global warming potential as a known greenhouse gas has made it undesirable when released into the atmosphere at a much higher pace than it is absorbed, as in the case of fossil fuels. This is not the case with biofuels. Their carbon footprint only accounts for the non-renewable carbon emissions associated with its cultivation, manufacture, or supply infrastructure. In the case of eucalyptus oil, most of the CO₂ emitted in the combustion is neutral, as it comes from the CO₂ absorbed by the tree during its growth. The valorization of leaves may inclusively avoid GHG emissions due to natural decomposition in nature. Therefore, it may be stated that the substitution of a given percentage of diesel fuel by eucalyptus oil nearly results in an equivalent percentage drop in GHG emissions associated with the blend. The CO₂ concentration found in the exhaust (displayed in Figure 7) should not be confused with the overall increase or decrease in the global warming potential of the fuel. This concentration varies according to the stoichiometry of the combustion for each fuel blend and the carbon atoms that were not involved in the complete combustion process, being released into the ambient space within the HC and CO molecules. In Figure 7, the measured carbon dioxide concentration reaches minimum values in all cases of engine speed and load. There is a trend of decreasing CO₂ concentration as the concentration of eucalyptus oil is higher in the fuel mixture. However, there are trends of increasing carbon dioxide for an engine speed of 1700 rpm and a load of 34.0%, where, with the presence of eucalyptus oil concentration in the fuel mixture, carbon dioxide becomes more prevalent in the exhaust gases. However, the values are at least 8% lower than the CO₂ emissions released by the reference fuel. Other researchers obtained similar results regarding carbon dioxide emissions [33].

It is noteworthy that, throughout the testing period, carbon monoxide (CO) was not detected by the analyzer. This result is explained by the fact that the diesel engine has a much higher air-to-fuel ratio compared to gasoline engines and employs a more efficient combustion process. The scale of the analyzer was not sufficiently fine as to detect any CO concentration besides under certain conditions, such as during cold starts.

3.2.3. Nitrogen Oxides

Emissions of nitrogen oxides are closely related to the combustion temperature, to the presence of excess air, and to the presence of oxygen in the fuel molecule. The exhaust gas temperature from blends of eucalyptus oil and diesel was generally lower for all engine speed regimes and all engine loads compared to the exhaust gas temperature of diesel. The nitrogen oxide emissions are presented in Figure 8. The general trend of emissions decreased as the concentration of biofuel increased. The performance level of an internal combustion engine can sometimes be evaluated indirectly based on nitrogen oxide emissions, as higher temperatures often indicate that combustion is more efficient and thus the engine itself is also operating more efficiently, generating more power. However, in the research presented, even when engine torque and brake thermal efficiency were higher, nitrogen oxide emissions remained lower than in the case of the reference fuel, by at least 17%, according to the measurements. This is a very good result when compared to other biofuels such as biodiesel, which tend to increase NOx emissions, namely due to the presence of oxygen in their molecule as reported in [23].

3.2.4. Smoke (Opacity)

Smoke emissions from a diesel engine are mostly associated with the quantity of visible solid particles released into the atmosphere because of the combustion process in the engine and were measured in terms of opacity, in percentage units. The quantity and composition of smoke emissions is one of the indicators of the effectiveness and efficiency of combustion. In Figure 9, the opacity of smoke measured for different engine loads and rotations with blends of biofuel (eucalyptus oil) and diesel is presented. As an overall observation, it may be seen that the addition of biofuel in percentages higher than 5% always brought down the opacity. This effect was especially strong in the case of high load and low speed where the smoke opacity of regular diesel was the highest. The opacity of smoke was reduced by approximately 57% for the 10EU90D blend compared to the reference fuel. Similar results were already presented in [16]. For the biofuel blends 20EU80D and 30EU70D, the opacity of smoke showed a slight increase compared to the others for the same load. This could be attributed to the viscosity of the fuel blend, which was slightly higher. However, even these blends achieved reductions in smoke opacity by approximately 20% compared to diesel. Lastly, it should be noted that eucalyptus essential oil contains oxygen, which facilitates combustion. Also, it has a higher cetane number, inducing better combustion. The lower level of smoke might be explained by these factors, and similar results of decreasing smoke opacity are also reported in works [28], where biofuels from renewable sources were used.

4. Conclusions

The main objective of the present study was to assess the potential use of eucalyptus oil, in blends with conventional fuel in a diesel engine. This evaluation targeted the engine’s performance parameters as well as the emissions generated, aiming to contribute to the reduction in fossil fuel consumption and associated pollutant and greenhouse gas emissions. Two different engine speeds (1700 and 2250 rpm), typical of light vehicle roads or motorway speeds, were tested. For each of these conditions, two engine loads corresponding to downhill or uphill driving were tested, providing four combinations of low and high speeds and engine loads.

Based on the analysis conducted, it is evident that torque, a crucial parameter for diesel engine performance, exhibited a significant variation based on the composition of fuel blends.

Biofuel blends containing eucalyptus oil demonstrated significant torque improvements at lower engine speeds and specific load conditions. The blend with 30% eucalyptus oil (30EU70D) exhibited the most substantial enhancement compared to diesel fuel, showing increases of 6.5% and 8.8% in torque at low speed/low load and low load/high speed settings, respectively. However, at higher engine speeds, these blends generally resulted in decreased torque, particularly pronounced for the 30EU70D blend at 2250 rpm under both load conditions. These findings highlight the nuanced performance characteristics of eucalyptus oil blends in diesel engines, suggesting potential benefits at lower speeds but challenges at higher operational ranges that warrant further investigation and optimization.

The 15% eucalyptus oil blend (15EU85D) exhibited a notable 7.3% decrease in fuel consumption compared to diesel. This finding indicates the potential for optimizing fuel efficiency under specific operational scenarios by selecting the appropriate biofuel blend compositions. The blend with 15% eucalyptus oil and 85% diesel (15EU85D) achieved the highest efficiency, approximately 7% greater than the reference fuel. These findings highlight the varying impacts of eucalyptus oil blends on engine efficiency under different operational conditions, suggesting potential benefits in specific contexts that warrant further investigation and optimization. Regarding the pollutant emissions, unburned hydrocarbon emissions were higher for biofuel blends compared to diesel fuel in the conditions of engine operation at low speed. However, these measurements, as the others, were made without exhaust after-treatment (e.g., catalytic converter), which would reduce these emissions that tend to be negligible in diesel engines.

• The CO concentration was so low in all of the tests that it was below the threshold measured by the gas analyzer, even without exhaust after-treatment. This low carbon monoxide (CO) concentration compared with gasoline engines can be attributed to the diesel engine’s constant operation with excess air, but CO emissions could still be detected under certain conditions, such as during cold starts.
• At specific conditions such as low engine speed and high engine load, CO₂ emissions showed an increase. Importantly, a significant aspect of CO₂ emissions from eucalyptus oil blends is their neutrality, as eucalyptus trees absorb CO₂ during their growth, effectively offsetting emissions associated with their combustion.
• Nitrogen oxide (NOx) emissions were lower for all mixtures of eucalyptus oil–diesel compared to diesel, indicating a decrease in combustion temperature, which was also indicated by the lower exhaust gas temperatures recorded when the eucalyptus oil concentration was higher in the mixture.
• The incorporation of eucalyptus oil blends into diesel fuel nearly always provided a reduction in smoke. This reduction was stronger (a peak 57% reduction in smoke opacity) for the case in which the reference fuel emitted the most smoke, at the low speed and high load setting. This behavior might be related to the higher cetane number of the eucalyptus-oil-based fuel, which improves combustion.

Results regarding the increased fuel consumption, decreased brake thermal efficiency, and lower smoke emissions were obtained in this study [28].

In the light of these obtained results, future research perspectives should focus on the optimization of injection management according to the specificities of each biofuel used, including blends of eucalyptus oil with diesel. It is essential to identify effective injection strategies to maximize engine performance and efficiency under various operating conditions. In addition, there is a clear direction towards increasing boost pressure to assess the full potential of these blends in terms of reduced emissions and increased fuel efficiency.

As a closing note, although the incorporation of eucalyptus oil was not always beneficial to engine behavior, the possibility of offsetting fossil fuel CO₂ emissions with the neutral, renewable CO₂ emissions of this biofuel, the good performance found in torque for several conditions, and in general the reduction in NOx and particulate matter emissions for most operating conditions suggest that this biofuel could be further investigated as a good candidate for the partial substitution of fossil fuels by sustainable renewable fuels. For sure, production costs, which are out of the scope of the present study, would play a critical role in the viability of this fuel substitution. Nevertheless, the present study, although limited in scope, seems to support the technical viability of energetically valorizing a byproduct of one of the most important industries from countries such as Portugal [7].