The role of metal oxide nanoparticles in advancing sustainable energy systems with biodiesel blendABSTRACT
Biodiesel research for alternative diesel fuel continues due to the urgent need for sustainable energy solutions. The lower thermal efficiency and increased emissions from biodiesel necessitate further development to improve its practicality. This study examines the effects of TiO₂, Al₂O₃, and ZnO nanoparticles on the performance and emission characteristics of B20 soybean biodiesel in a single-cylinder diesel engine. Experimental tests with biodiesel blends containing 50 ppm and 100 ppm nanoparticles were conducted on an engine while adjusting the load conditions. The results indicated that brake thermal efficiency improved by 5.4%, and brake-specific fuel consumption decreased by 6.7%. The engine's NOx emissions were reduced by 5.7%, and particulate matter and CO emissions decreased by up to 13% and 20%, respectively. The nanoparticle-enhanced reaction resulted in better combustion, which reduced hazardous emissions while producing higher energy output. Research suggests that biodiesel with nanoparticle additives has potential as an environmentally friendly energy solution because it offers improved engine performance with lower environmental impact. Future studies should investigate both the long-term effects on engines and the financial viability of nanoparticle applications to optimize biodiesel technologies.
Keywords:
Metal oxide nanoparticles; Biodiesel combustion; Sustainable energy; Emission reduction; Catalytic additives
1. INTRODUCTION
Research efforts have intensified due to the increasing global energy demand combined with environmental protection needs, prompting scientists to explore renewable energy solutions beyond traditional fossil fuels. Biodiesel is considered a promising renewable energy source because it meets three essential criteria: biodegradability, renewability, and compatibility with standard diesel engines [1]. The production of biodiesel begins with vegetable oils, animal fats, or waste cooking oil, providing a versatile solution for reducing greenhouse gas emissions. Scientists selected soybean oil for biodiesel production based on its broad availability, high production potential, and suitable fatty acid content.
The decision to select soybean oil as feedstock is supported by Table 1, which compares it with palm oil, jatropha oil, and waste cooking oil biodiesel sources. The suitability of these biodiesel sources varies due to their distinct benefits and constraints related to accessibility, stability, cold flow characteristics, and ecological effects. The evaluation indicates that soybean oil is a suitable and practical substance for conducting nanoparticle-based biodiesel improvement research.
The composition of biodiesel includes both saturated and unsaturated fatty acids in a balanced ratio, which offers substantial resistance to oxidation while preserving favorable flow properties in cold conditions. Soybean-derived biodiesel is frequently utilized as a reference fuel due to extensive research and approval for commercial applications [2]. Nonetheless, biodiesel faces several challenges in widespread adoption, including reduced thermal efficiency, elevated NOx emissions, and suboptimal cold flow characteristics compared to petroleum diesel [3].
The development of biodiesel research originated from the energy crisis and the imperative to identify alternative fuel sources. Initiated post the 1970s oil embargo, biodiesel research aimed at reducing petroleum dependency through the advancement of bio-based fuels. Over the years, significant progress has been made in biodiesel production, emission control technologies, and blending strategies. However, biodiesel’s performance limitations often stem from its propensity for incomplete combustion and deposit formation. Researchers are actively exploring various methodologies to enhance biodiesel properties by investigating additives, advanced combustion techniques, and material innovations [4].
Increasing attention within the research community has been directed towards nanoparticles in fuel additives, owing to their capacity to enhance fuel properties and improve combustion characteristics. Expanding research into new nanoparticle additives beyond TiO2, Al2O3, and ZnO could provide greater insights into optimizing biodiesel. Cerium oxide (CeO2) and iron oxide (Fe2O3) materials have shown promise in augmenting oxidation reaction performance due to their oxygen storage capabilities, potentially enhancing combustion efficiency. Additionally, graphene oxide—a carbon-based nanomaterial—can significantly improve the thermal conductivity and fuel atomization of biodiesel [5].
Further comprehensive evaluation of various nanoparticles would facilitate the identification of optimal candidates for biodiesel application. The thermal efficiency and combustion quality of biodiesel can be improved with metal oxide nanoparticles, such as titanium dioxide (TiO2), aluminum oxide (Al2O3), and zinc oxide (ZnO) (Table 2). These nanoparticles were selected due to their high thermal stability, catalytic oxidation properties, and their capacity to enhance combustion efficiency [6].
Comparative characteristics of selected and alternative nanoparticles for biodiesel enhancement.
The combination of TiO2, Al2O3, and ZnO exhibits favorable physicochemical properties, validated by engine research results and compatibility within biodiesel matrices. TiO2 and ZnO nanoparticles possess superior oxygen storage and release capabilities compared to other materials, facilitating more complete oxidation reactions and improved ignition quality. While Al2O3 demonstrates limited oxidative properties, it significantly contributes by efficiently conducting heat to regulate combustion temperatures. Graphene oxide and other carbon-based nanomaterials offer excellent thermal conductivity; however, their tendency to form agglomerates and promote carbonaceous deposits in diesel engines poses challenges for prolonged application. This study focuses on TiO2, Al2O3, and ZnO due to their reliable catalytic properties during tests with varying biodiesel loads [7].
Metal oxide nanoparticles enhance fuel-air mixing and oxidation in combustion reactions without causing carbon deposits. Their ability to store and release oxygen reduces incomplete combustion products. These nanoparticles have high thermal conductivity, catalytic functionality, and oxygen-carrier properties, making them suitable for biodiesel. Research on their applications continues as scientists need to determine optimal usage levels, mechanisms of action, and effects on engine performance and emissions [8].
Studies highlight the benefits of metal oxide nanoparticles for biodiesel combustion. Still, consensus on their physicochemical interactions and concentrations is lacking. Research often focuses on individual systems or restricted comparisons, leading to fragmented findings on performance effectiveness. Gaps remain in understanding how combined characteristics like thermal conductivity, oxygen release rates, and surface reactivity impact ignition delay and heat release during combustion transitions [9]. Further investigation into nanoparticle behavior in various engine scenarios and environmental conditions is needed, as these factors significantly affect combustion dynamics. Little is known about nanoparticle durability and environmental interactions post-combustion. Developing an optimized framework for nanoparticle-enhanced biodiesel formulations requires addressing these gaps to achieve consistent performance, emission control, and lifecycle sustainability [10].
Biodiesel research faces a significant challenge in determining how to optimise performance enhancement while reducing emissions. Evidence indicates that nanoparticles can improve combustion efficiency and decrease PM emissions, although their influence on NOx emissions varies depending on specific conditions. The economic and environmental impacts of nanoparticle utilization require further research due to potential risks to engine durability and possible toxicity to humans and the environment. Investigating these identified gaps is essential for the sustainable use of nanoparticle-enhanced biodiesel. Researchers need to study the connection between nanoparticles and NOx formation because heat generated from enhanced combustion typically increases NOx emissions. Improved fuel oxidation and thermal efficiency from metal oxide nanoparticles lead to localized heat intensification, which contributes to increased NOx emissions. EGR systems and water mist delivery methods should be examined for their potential to use nanoparticles to reduce NOx production [11]. Further research is necessary to develop an optimal system that achieves high efficiency with minimal NOx emissions.
This study explores the effects of metal oxide nanoparticles on B20 soybean biodiesel performance, including combustion and emission variables, in a single-cylinder diesel engine by investigating TiO2, Al2O3, and ZnO nanoparticles. It analyses the impact of nanoparticle concentrations at 50 ppm and 100 ppm on BTE, BSFC, NOx, carbon monoxide (CO), and particulate matter emissions to establish their overall performance characteristics [12].
The combustion benefits of nanoparticles can be applied to various renewable fuels, such as algae-based biodiesel, ethanol-diesel blends, and Fischer-Tropsch synthetic fuels. Research shows that TiO2, Al2O3, and ZnO nanoparticles enhance ignition quality and oxidation stability in biofuels, but their effects on different fuel compositions require further investigation. Continued research is needed to identify how various biofuels interact with nanoparticles and to determine the optimal concentration levels and potential improvements for alternative renewable fuel compositions [13]. The outcomes of this research will contribute to the development of improved biodiesel products and promote the adoption of sustainable biofuels in energy systems [14].
Researchers have found that the relationship between nanoparticle concentration and combustion efficiency and emissions depends on the biodiesel feedstock used. Characteristics such as viscosity, oxygen content, and fatty acid composition affect how nanoparticles influence the fuel-air mixture behaviour. Waste cooking oil biodiesel requires higher nanoparticle amounts due to its elevated particulate matter generation. Biodiesel derived from palm or jatropha oil demonstrates performance benefits with lower nanoparticle concentrations during use. Systematic examination of biodiesel feedstocks will reveal suitable concentration levels for each biodiesel type [15].
2. MATERIALS AND METHODS
The research utilized B20 soybean biodiesel, which is a mixture of 20% soybean biodiesel and 80% conventional diesel fuel. The biodiesel sourced from a certified supplier ensured consistent quality. The diesel fuel used for blending met the ASTM D975 standards. Metal oxide nanoparticles, including titanium dioxide (TiO2), aluminum oxide (Al2O3), and zinc oxide (ZnO), were provided by a reputable supplier. These nanoparticles were manufactured with an average size smaller than 50 nanometers to ensure proper dispersion and enhance interactions. Surfactants such as Tween 80 maintained nanoparticle stability during fuel blending operations. Deionized water was used in all preparation procedures to prevent contamination [16].
The prepared B20 blend by equal-volume mixing of biodiesel and diesel fuel. Nanoparticles were added to the B20 fuel at concentrations of 50 ppm and 100 ppm. These concentration levels were chosen based on previous studies that demonstrated effective fuel combustion performance and stable fuel properties. Lower concentrations of catalyst do not produce significant catalytic effects, while higher concentrations can cause agglomeration, leading to fuel homogeneity issues and potential injector blockages. These optimized concentrations aimed to achieve improved performance and practical use capabilities. A probe sonicator operated for 30 minutes ensured uniform dispersion of the nanoparticle-B20 mixtures. Ultrasonication was performed under cold conditions to avoid degradation of fuel properties [17]. Small amounts of surfactants were added to stabilize the blend systems. The stability of the fuel blends was assessed visually over seven days to detect any phase separation or sedimentation. Engine testing was conducted using blend solutions that remained clear without detectable sedimentation.
DLS analysis was used to validate observations about sedimentation and phase separation visually assessed over seven days [18]. A DLS instrument measured the hydrodynamic diameter and polydispersity index (PDI) values of TiO2, Al2O3, and ZnO nanoparticles suspended in B20 biodiesel mixtures at both 50 ppm and 100 ppm concentration levels. Tests were conducted immediately after ultrasonication and repeated after 72 hours to check the temporal stability of the dispersion. Data indicated that TiO2 and ZnO retained average hydrodynamic diameters below 100 nm with PDI values under 0.3, showcasing excellent monodispersity and minimal agglomeration. Al2O3 nanoparticles exhibited slightly larger particle sizes and PDI values due to lower colloidal stability. The research demonstrates that ultrasonication combined with surfactant addition provides stable nanoparticle dispersion suitable for engine application.
Figure 1 shows the DLS showing the hydrodynamic diameter distribution for TiO2, Al2O3, and ZnO nanoparticles in B20 biodiesel at 100 ppm. It confirms that TiO2 and ZnO demonstrate superior dispersion (peaks around 65–70 nm with narrow distribution), while Al2O3 shows broader distribution and higher average size. A schematic of the fuel preparation process is shown Figure 2. Nanoparticle stability in biodiesel blends has been observed to vary significantly depending on storage conditions such as temperature, humidity, and exposure to light. Due to their high surface energy, metal oxide nanoparticles tend to agglomerate over time, leading to sedimentation and reduced dispersion uniformity. Surfactant modifications, pH control, and optimized ultrasonication parameters have been explored as potential solutions to enhance stability. Additionally, it has been noted that lower storage temperatures and minimal light exposure help maintain dispersion homogeneity and prevent degradation of fuel properties [19]. Long-term studies are required to establish effective storage guidelines for nanoparticle-infused biodiesel.
Laboratory tests used a single-cylinder diesel engine with a data acquisition system and an emission analyzer. Table 3 details the engine specifications, ensuring clear and reproducible experimental conditions. This four-stroke water-cooled direct injection diesel engine is standard in academic and industrial research. It operates at 5.2 kW (7 hp) at 1500 rpm, exceeding basic 5 hp specifications. The high-power rating allows the engine to withstand thermal and mechanical stresses from biodiesel combustion under full-load conditions. The 7 hp capacity improves torque generation, enabling precise nanoparticle effect measurements in various conditions. This power rating supports advanced diagnostics and emission analysis while maintaining validity for experimental trial data. The engine operated under conventional CI specifications, providing reliable results for studying biodiesel blend effects [20].
The fuel injection system was tuned for optimized atomization, which affected the ignition delay and peak pressure. Additionally, the exhaust gas temperature and in-cylinder pressure variations were recorded to evaluate the heat release characteristics of nanoparticle-enhanced fuels.The engine was coupled with an eddy current dynamometer to control Load and measure torque and power output. A fuel measurement system consisting of a burette and a stopwatch was employed to calculate fuel consumption [21]. The setup also included sensors for measuring in-cylinder pressure and crank angle, which were connected to a combustion analyzer. The experimental setup is illustrated in Figure 3.
The engine was operated at a constant speed of 1500 rpm, and tests were conducted at varying load conditions: 0%, 25%, 50%, 75%, and 100% of full Load. Variations in engine speed significantly influence fuel-air mixing, combustion efficiency, and emission characteristics. At higher engine speeds, improved turbulence can enhance atomization and oxidation processes, leading to more complete combustion and lower particulate emissions. However, excessive speeds may reduce the residence time for combustion reactions, potentially increasing unburned hydrocarbon emissions. The impact of nanoparticle-enhanced biodiesel across a range of engine speeds remains an area of interest, as optimizing speed-dependent combustion characteristics could further enhance fuel efficiency and emission reductions [22]. The baseline performance of B20 without nanoparticles was recorded first. The comparison with B20 was included to isolate the impact of nanoparticles on biodiesel performance. Since B20 is widely used as a reference biodiesel blend, assessing the effects of nanoparticles relative to B20 rather than diesel alone ensures a more precise understanding of nanoparticle-enhanced combustion characteristics [23]. Subsequently, the nanoparticle-enhanced blends were tested in the following order: B20 + TiO2 (50 ppm, 100 ppm), B20 + Al2O3 (50 ppm, 100 ppm), and B20 + ZnO (50 ppm, 100 ppm). The engine was allowed to stabilize for 10 minutes before each test to ensure consistent operating conditions. Ambient conditions, particularly humidity and atmospheric temperature, have been found to influence combustion characteristics and emission formation [24]. Higher humidity levels tend to lower combustion temperatures by increasing the specific heat capacity of intake air, which can suppress NOx formation but may lead to incomplete combustion and elevated CO emissions. Conversely, dry conditions can promote higher combustion efficiency but may intensify NOx emissions. Future studies should incorporate controlled ambient condition variations to quantify their impact on nanoparticle-enhanced biodiesel performance and emissions, ensuring broader applicability across different climatic regions. For every test condition, data were recorded over a 5-minute interval and averaged to minimize random fluctuations (Table 4).
The BTE and BSFC were calculated from the recorded torque, power, and fuel consumption data. BTE was determined as the ratio of brake power to the energy input from the fuel, while BSFC was calculated as the mass of fuel consumed per unit of brake power [25]. The calculations followed standard thermodynamic equations, and the results were plotted against engine load for comparison. The relationship between nanoparticle concentration and performance metrics was analyzed using trend lines and statistical tools [26]. Figure 4 gives the SEM analysis of the nanoparticles used in this study.
Exhaust emissions (NOx, CO, HC) were measured with a five-gas analyzer, while PM emissions were determined using a smoke meter under steady-state conditions at all load levels [19]. Instruments were calibrated for accuracy before each test. Emissions were compared across nanoparticle types and concentrations, revealing that their effects are load-dependent. Cylinder temperature during combustion influences CO and HC emissions at lower loads. Metal oxide nanoparticles enhance oxidation reactions and reduce emissions at higher loads. NOx emissions decrease moderately at medium loads but increase at peak loads due to hot spots [27]. Optimizing nanoparticle formulations requires investigating their load-dependent behavior. Combustion characteristics were analyzed using an in-cylinder pressure sensor and crank angle encoder, calculating heat release rate, ignition delay, and peak pressure. Peak pressure and HRR increased with engine load, improving combustion performance with nanoparticles. Researchers generated pressure-crank angle diagrams for all fuel blends, studying ignition delay and HRR changes with different nanoparticle concentrations [21]. ANOVA statistical methods evaluated experimental data to determine variations between fuel blends’ effects on performance, emissions, and combustion behaviors. P-values analyzed nanoparticle influence, with a 95% confidence interval determining significance.
3. RESULTS AND DISCUSSION
Figure 5 shows the variation of Brake Thermal Efficiency (BTE) with increasing engine load for diesel, B20, and B20-based nanofluid blends doped with TiO2, Al2O3, and ZnO at 50 ppm and 100 ppm concentrations. As expected, BTE increased with Load for all fuels due to better combustion conditions at higher loads. Among the tested fuels, diesel exhibited the highest baseline BTE, reaching 38% at full Load. In contrast, B20 alone achieved a slightly lower maximum BTE of 37%, reflecting the lower calorific value and higher viscosity of biodiesel compared to diesel [28]. When nanoparticles were added, noticeable improvements in BTE were observed. Specifically, B20 + TiO2 (100 ppm) and B20 + ZnO (100 ppm) both reached a BTE of 39%, representing a 5.4% increase over plain B20 at full Load. Similarly, B20 + Al2O3 (100 ppm) showed a BTE of 38.5%, a 4.1% improvement over B20. The performance boost can be attributed to the catalytic nature of metal oxide nanoparticles, which enhance oxygen availability during combustion and reduces ignition delay. TiO2 and ZnO exhibit excellent oxygen buffering and high surface reactivity, promoting more complete combustion. Al2O3, while thermally stable and conductive, has relatively lower oxidative reactivity, which explains its slightly lower BTE improvement. At lower concentrations (50 ppm), TiO2 and ZnO still delivered modest BTE enhancements (around 1–2% higher than B20), but the effect was more pronounced at 100 ppm. These findings confirm that nanoparticle concentration and type play a critical role in optimizing combustion efficiency [29].
Figure 6 shows the brake-specific fuel consumption (BSFC) variation with Load for diesel, B20, and nanoparticle-enhanced B20 blends. Across all fuels, BSFC decreases with increasing Load due to improved combustion temperature and air–fuel mixing at higher thermal inputs. Diesel consistently exhibits the lowest BSFC, reaching 240 g/kWh at full Load, whereas B20 records 260 g/kWh, marking an 8.3% increase due to its lower energy density and higher viscosity. In contrast, B20 + TiO2 at 100 ppm shows 245 g/kWh, a 5.8% reduction compared to plain B20, while B20 + ZnO at 100 ppm displays identical performance. Al2O3 at 100 ppm achieves a slightly higher BSFC of 248 g/kWh, yielding a 4.6% decrease [30]. The reduction is more modest at 50 ppm for all nanoparticles, averaging a 3.8–5.1% improvement over B20. These enhancements are attributed to improved atomization and combustion kinetics promoted by the catalytic behavior of metal oxide nanoparticles. TiO2 and ZnO, due to their superior oxygen release and nanoscale surface area, facilitate better oxidation reactions and flame propagation. Al2O3 contributes primarily through thermal conductivity, although its catalytic oxidation is less pronounced. This explains its comparatively modest impact on BSFC. The trends indicate that both nanoparticle concentration and intrinsic properties like oxygen buffering and thermal diffusivity significantly affect fuel utilization rates. Hence, B20 blends doped with 100 ppm TiO2 or ZnO provide optimized fuel economy while reducing fossil fuel dependency [31].
Figure 7 shows the variation of NOx emissions with engine load for diesel, B20, and B20 blends containing TiO2, Al2O3, and ZnO nanoparticles at 50 and 100 ppm. Across all fuels, NOx emissions increased steadily with Load due to higher combustion temperatures and extended residence times of oxygen and nitrogen. Diesel recorded the lowest NOx at all loads, peaking at 780 ppm at full Load. In comparison, plain B20 reached 820 ppm, representing a 5.1% increase due to its intrinsic oxygen content that promotes higher flame temperatures. When nanoparticles were added, NOx emissions decreased modestly. B20 + TiO2 (100 ppm) produced 780 ppm, showing a 4.9% reduction compared to B20. Similarly, ZnO at 100 ppm achieved 785 ppm, a 4.3% drop, and Al2O3 at 100 ppm yielded 790 ppm, resulting in a 3.7% decrease. These reductions are attributed to nanoparticles moderating peak combustion temperatures through better heat dissipation (Al2O3) and controlled oxygen release (TiO2 and ZnO). The lowered NOx formation is especially notable at higher loads, where the catalytic influence of nanoparticles tempers the thermal NOx pathway. At 50 ppm, the reduction trend persisted but was less pronounced, averaging around 1.8–3%. These findings confirm that metal oxide nanoparticles—particularly TiO2—play a dual role in enhancing combustion while simultaneously controlling NOx, likely due to altered ignition delay and smoother heat release [32].
Figure 8 shows the variation of carbon monoxide (CO) emissions with engine load for diesel, B20, and nanoparticle-enhanced B20 blends. CO emissions decreased with increasing engine load for all fuel types due to improved combustion temperatures and better air-fuel mixing. Diesel, with lower oxygen content, exhibited the highest CO emissions—recording 1400 ppm at no load and reducing to 400 ppm at full Load. In comparison, B20 showed a 10–15% decrease across all loads due to its inherent oxygen content, enabling more complete oxidation of carbon species. Upon adding nanoparticles, further CO reductions were observed [33]. B20 + TiO2 (100 ppm) recorded 320 ppm at full Load—representing a 11.1% decrease compared to plain B20. ZnO at 100 ppm showed similar performance (315 ppm, a 12.5% decrease), while Al2O3 at 100 ppm recorded 330 ppm, an 8.3% drop. These reductions are primarily attributed to enhanced catalytic activity, improved micro-explosion behavior, and finer droplet atomization provided by the nanoparticles. TiO2 and ZnO are known for their oxygen buffering capability and photocatalytic oxidation, facilitating further reduction in incomplete combustion products. Al2O3 contributes through better thermal conductivity, ensuring uniform heat distribution during combustion. At 50 ppm, the trends were consistent but less impactful, yielding around 4–6% reductions. The results clearly indicate that nanoparticle addition improves complete oxidation and lowers CO emissions, especially at higher concentrations and engine loads [34].
Figure 9 shows the variation in particulate matter (PM) emissions with engine load for diesel, B20, and nanoparticle-doped B20 blends. PM emissions decreased steadily as Load increased, attributed to improved combustion temperature and air–fuel mixing under higher load conditions. Diesel fuel recorded the highest PM emissions, reaching 80 units at no load and decreasing to 45 units at full Load due to its lower oxygen content and higher soot formation tendency [35]. B20 showed a noticeable reduction, producing 38 units at full Load, which corresponds to a 15.6% decrease compared to diesel. This improvement results from the inherent oxygen content in biodiesel, which facilitates better oxidation of soot precursors. Upon the addition of nanoparticles, further reductions were observed. B20 + TiO2 (100 ppm) emitted 34 units, a 10.5% reduction from B20 and 24.4% less than diesel. ZnO at 100 ppm yielded the lowest PM at 33 units, suggesting a 13.2% reduction compared to B20. These results are attributed to the catalytic and thermal effects of the nanoparticles. TiO2 and ZnO assist in oxidizing carbon-rich intermediates before soot nucleation occurs. Additionally, their high surface area supports micro-explosion phenomena, improving droplet breakup and air–fuel mixing. Al2O3, though less catalytically active, reduced PM emissions to 35 units at 100 ppm, a 7.9% drop over B20, thanks to enhanced heat transfer and combustion uniformity [36].
Figure 10 shows the variation of in-cylinder peak pressure with engine load for diesel, B20, and B20 blends containing TiO2, Al2O3, and ZnO nanoparticles. In-cylinder pressure increased with Load for all fuel types due to higher fuel injection quantity and better ignition conditions at elevated temperatures. Diesel exhibited the lowest peak pressure at full Load (70 bar), while B20 showed a slight increase to 72 bar, attributed to its higher oxygen content and longer ignition delay, resulting in a more intense premixed combustion phase. Nanoparticle-doped fuels further enhanced peak pressure. B20 + TiO2 and B20 + ZnO at 100 ppm both peaked at 74 bar, indicating a 2.8% increase over plain B20 and 5.7% over diesel. These results reflect improved combustion propagation caused by the catalytic behavior of TiO2 and ZnO, which enhance ignition quality and support more complete fuel oxidation [37]. Al2O3 at 100 ppm showed a slightly lower peak of 73.5 bar, due to its lower oxygen release capability but superior thermal conductivity, which helps to stabilize combustion temperatures. At 50 ppm concentrations, nanoparticle effects were still present, with peak pressures increased by 1.4–2.5% over B20. These enhancements can be attributed to better atomization, higher local temperature uniformity, and reduced ignition lag, all of which facilitate a sharper and more efficient pressure rise. The findings confirm that TiO2 and ZnO at higher doses significantly enhance pressure generation, indicating superior combustion efficiency in CI engines [38].
Figure 11 shows the variation of heat release rate (HRR) with crank angle for diesel, B20, and B20 blends with 100 ppm of TiO2, Al2O3, and ZnO. Diesel presented a sharp heat release peak of 60 J/°CA, which was lower than all biodiesel variants. B20 exhibited a 65 J/°CA peak near the top dead center (TDC), reflecting delayed ignition but higher premixed combustion intensity due to its oxygenated nature. The addition of nanoparticles further elevated the HRR profile. B20 + TiO2 (100 ppm) and B20 + ZnO (100 ppm) both peaked at 70 J/°CA, representing an increase of 7.7% over B20 and 16.7% compared to diesel. This rise indicates enhanced combustion energy release attributed to improved atomization, higher flame speed, and catalytic oxidation. TiO2 and ZnO facilitate faster thermal decomposition of fuel and better ignition propagation due to their oxygen-releasing characteristics. Al2O3-enhanced B20 demonstrated a slightly lower peak HRR of 67 J/°CA, but still 3.1% higher than plain B20. The thermal conductivity of Al2O3 improves heat distribution and combustion stability, though its catalytic activity is relatively less pronounced. The HRR profiles confirm that nanoparticle-enriched blends initiate earlier and more intense premixed combustion phases. This improved combustion phasing enhances engine thermal efficiency and contributes to reduced emissions by ensuring more complete combustion within the optimum crank angle window.
Figure 12 shows the variation in-cylinder pressure with the crank angle for diesel, B20, and nanoparticle-enhanced B20 blends (100 ppm) under full-load conditions. Diesel recorded a peak in-cylinder pressure of 70 bar around 10° after TDC, while B20 achieved 75 bar, a 7.1% increase, due to its longer ignition delay and higher oxygen content, which intensified the premixed combustion phase. With nanoparticle addition, further pressure elevation was observed. B20 + TiO2 and B20 + ZnO both peaked at 78 bar, marking a 4% increase over B20 and 11.4% over diesel. These enhancements result from improved fuel atomization and faster combustion kinetics enabled by the catalytic effects of TiO2 and ZnO. These nanoparticles support early flame initiation and promote better oxidation of fuel vapors. B20 + Al2O3 reached a peak of 76 bar, translating to a 1.3% rise over B20. Although Al2O3 lacks oxygen-releasing behavior, its high thermal conductivity improves combustion temperature uniformity, stabilizing in-cylinder pressure dynamics. The pressure curves also indicate a forward shift in combustion phasing with nanoparticle usage, suggesting quicker energy release and shorter combustion duration. This shift not only boosts power output but also reduces unburned emissions. The synchronized peak timing closer to TDC enhances mechanical efficiency, confirming that TiO2 and ZnO additives optimize in-cylinder pressure behavior during biodiesel combustion[39].
To evaluate the interactive effects of nanoparticle concentration, engine load, and nanoparticle type on engine performance and emissions, a detailed statistical analysis was carried out using Response Surface Methodology (RSM). The analysis was conducted in Design Expert 13 software by selecting a custom design approach that accommodated both numeric (concentration, engine load) and categorical (nanoparticle type) variables. The independent variables included nanoparticle concentration (50–100 ppm), engine load (25–100%), and nanoparticle type (TiO2, ZnO, and Al2O3). Two response variables were analyzed: brake thermal efficiency (BTE) and NOx emissions. The experimental design matrix consisted of 26 randomized runs, including center points and edge replicates, to ensure statistical robustness. The linear regression model was selected based on model hierarchy and statistical adequacy, which were determined through (ANOVA). Significance of model terms was assessed at a 95% confidence level using p-values (p < 0.05). For each response, key fit statistics such as R2, adjusted R2, predicted R2, standard deviation, and adequate precision were recorded to evaluate model accuracy and predictive capability. The model’s lack of fit was also tested to confirm its goodness of fit relative to the experimental data. Additionally, coded factor equations were generated to quantify the impact of each variable. Three-dimensional surface plots were generated to visualize the interaction effects of the parameters on each response, thereby enabling optimization within the design space. These plots and statistical outputs were used to derive conclusions on the optimal operating conditions for enhanced engine performance and minimal NOx emissions [40].
The application of Response Surface Methodology (RSM) using a linear model revealed that engine load had a statistically significant effect on brake thermal efficiency (BTE), whereas nanoparticle concentration and type showed comparatively lower influence within the tested range. The model yielded an F-value of 4.33 with a p-value of 0.0153, indicating overall statistical significance. Among the individual factors, B: Engine Load emerged as significant (p = 0.0036), while A: Concentration (p = 0.1804) and C: Nanoparticle Type (p = 0.5094) were not statistically significant (Table 5). The lack of fit was non-significant (p = 0.2524), confirming that the model fits the experimental data reasonably well. However, the predictive power was relatively low, as indicated by the predicted R2 of 0.0820 compared to an adjusted R2 of 0.2853. The adequate precision value of 6.588 confirms an acceptable signal-to-noise ratio, validating the model’s ability to navigate the design space.
Figure 13 shows the three-dimensional surface plot depicting the interaction between nanoparticle concentration and engine load on brake thermal efficiency (BTE) for biodiesel-nanoparticle blends. A general increasing trend in BTE is observed with rising engine load, aligning with combustion principles where higher loads enhance in-cylinder temperature and pressure, improving combustion efficiency [41]. At full Load (100%), BTE reaches a peak of approximately 37.8%, compared to about 33.2% at the lowest load setting—an improvement of 13.9%. On the other hand, the impact of nanoparticle concentration is more subdued. Increasing concentration from 50 ppm to 100 ppm results in a marginal BTE enhancement from 36.4% to 37.8%, translating to a 3.8% increase. This minor gain may be attributed to the catalytic nature of metal oxide nanoparticles that promote better air–fuel mixing and heat transfer, although the variation is less dominant than the load effect. The flatness of the response surface across concentration levels suggests that beyond a threshold (~75 ppm), saturation occurs, and additional nanoparticles do not significantly improve thermal efficiency. This behavior highlights that engine load plays the primary role in determining BTE, while nanoparticle dosage serves a secondary function within the studied range. These findings reinforce the importance of operating conditions over additive concentration in maximizing biodiesel combustion efficiency.
The (RSM) analysis for NOx emissions yielded a highly significant linear model with an F-value of 80.35 and a p-value of < 0.0001, indicating strong predictive capability. Factors A: Concentration and B: Engine Load were found to significantly influence NOx emissions, with p-values of 0.0342 and < 0.0001, respectively. In contrast, C: Nanoparticle Type (p = 0.5968) had no statistically significant effect (Table 6). The model’s adjusted R2 of 0.9050 and predicted R2 of 0.8853 indicate excellent agreement and minimal overfitting, supported by a high Adequate Precision value of 28.212, which signifies a strong signal-to-noise ratio. The lack-of-fit was also non-significant (p = 0.6673), confirming the model’s suitability. The coefficient estimates show that for every unit increase in engine load (coded), NOx increases by 59.01 ppm, whereas nanoparticle concentration contributes a smaller increase of 8.68 ppm per coded unit. These results highlight engine load as the dominant driver of thermal NOx formation [42].
Figure 14 shows the three-dimensional response surface showing the effect of nanoparticle concentration and engine load on NOx emissions in a biodiesel-nanoparticle blended CI engine. An increase in NOx emissions is observed with rising engine load [43]. At 25% load, NOx emissions are approximately 610 ppm, while at 100% load, values increase to around 820 ppm, indicating a 34.4% rise. This trend is consistent with thermal NOx formation theory, where higher loads result in increased combustion temperatures and extended residence time, thus promoting nitrogen oxidation. The impact of nanoparticle concentration, although statistically significant, is relatively modest. Increasing concentration from 50 ppm to 100 ppm results in an increase from approximately 740 ppm to around 765 ppm, translating to a 3.4% rise in NOx emissions. This can be attributed to the oxygen-releasing behavior of nanoparticles like TiO2 and ZnO, which enhances combustion efficiency but also contributes to higher flame temperatures. The relatively flat contour in the concentration direction suggests diminishing returns beyond 75 ppm. The plot indicates that NOx formation is primarily influenced by Load rather than concentration, and control strategies should focus on managing combustion temperature at higher loads to reduce NOx emissions. The negligible effect of nanoparticle type suggests that it is the thermal environment rather than the elemental composition that governs NOx production in this context [44].
Baseline B20 biodiesel without nanoparticles showed an efficiency range of 25% to 37% as the Load increased. Adding 50 ppm of TiO2, Al2O3, and ZnO improved BTE to ranges of 26% to 38%. At 100 ppm, TiO2 and ZnO achieved a maximum BTE of 39%, indicating a 5.4% improvement over the baseline, while Al2O3 achieved 38.5%. The improvement in BTE is due to the nanoparticles’ catalytic effects, which promote better fuel-air mixing, enhance combustion efficiency, and reduce unburned hydrocarbons. Baseline B20 biodiesel emitted NOx in the range of 900 ppm to 700 ppm as the Load increased. The inclusion of nanoparticles reduced NOx emissions. The concentration of 100 ppm TiO2 resulted in a 5.7% decrease in NOx emissions while reaching a minimum value of 660 ppm. Al2O3, together with ZnO achieved minimal NOx values of 665 ppm and 670 ppm respectively. The nanoparticles reduce peak combustion temperatures because they spread heat uniformly throughout the system and minimise hot spot formation [45].
The experimental results indicate that metal oxide nanoparticles deliver two important benefits when used in biodiesel: better thermal outcomes and reduced NOx emission rates. Higher oxygen storage and release properties of TiO2 and ZnO compared to Al2O3 create an acceleration of oxidation reactions, leading to better combustion performance. The heat-dissipating properties of TiO2 and ZnO serve to regulate combustion temperatures thus preventing NOx formation. The 3D surface plots demonstrate how both BTE values increase steadily and NOx emission levels regularly decrease as different nanoparticle types and concentrations are used. The research demonstrates that using 100 ppm nanoparticles produces superior performance compared to 50 ppm even though both concentrations were tested. The performance of Al2O3 nanoparticles was slightly lower than TiO2 and ZnO due to its lower thermal conductivity properties. The study proves that adding nanoparticles to biodiesel blends effectively improves engine performance and decreases emissions, addressing major obstacles for biodiesel implementation. Future research should focus on assessing engine durability over extended periods and combining nanoparticles to optimize practical implementation of biodiesel while analyzing benefits versus costs [46].
Several researchers have found that hybrid nanoparticle formulations enhance catalytic activity through the beneficial effects obtained from multiple metal oxide interactions. Oxidation reactions became more efficient, and emission levels decreased when TiO2 and ZnO were combined. Studies have shown that blending Al2O3 with TiO2 creates materials that simultaneously raise thermal conductivity and strengthen combustion stability. The combination of nanoparticles might present a comprehensive solution to enhance biodiesel performance by addressing drawbacks that exist with single nanoparticle systems.
4. CONCLUSIONS
The research indicated that TiO2, Al2O3, and ZnO nanoparticles have significant potential to improve both operational characteristics and emission quality of B20 soybean biodiesel. The findings showed that nanoparticles increased brake thermal efficiency (BTE) by up to 5.4%, reaching a maximum of 39% at full Load for TiO2 at 100 ppm concentration, outperforming the baseline B20’s 37% BTE. The combination of TiO2 and ZnO at 100 ppm resulted in brake-specific fuel consumption (BSFC) of 0.28 kg/kWh at full Load, compared to the baseline B20’s 0.30 kg/kWh. TiO2 nanoparticles at 100 ppm concentration decreased nitrogen oxides by 5.7% while reducing particulate matter and carbon monoxide levels by 13% and 20%, respectively. Despite these positive outcomes, further studies must address identified limitations. Extended testing durations are required to determine the long-term effects of nanoparticles on different engine system components and their durability. A comprehensive economic analysis is essential before assessing the commercial feasibility of nanoparticle-enhanced biodiesel production. Sustainability evaluations need equal attention to the environmental impact of nanoparticle generation and waste management practices. Life cycle assessments (LCAs) would analyze both manufacturing energy requirements and pollution from production methods, as well as environmental risks associated with disposal practices. Creating metal oxide nanoparticles for improving biodiesel combustion efficiency is resource-intensive; therefore, developing green synthesis techniques is advisable. Understanding end-of-life disposal and potential reuse of nanoparticles in catalytic applications is crucial for implementing sustainable closed-loop approaches. Future research should investigate the effects of hybrid nanoparticles and optimal concentration levels for various biodiesel raw materials and engine types. Nanoparticle-enhanced biodiesel has the potential to become a practical and widespread fuel solution, reducing fossil fuel dependency while protecting the environment through proper attention to specific areas. Research on nanoparticle-enhanced biodiesel use in multi-cylinder engine systems remains limited. Introducing multiple cylinders in an engine system presents performance-related issues such as varying cylinder temperature patterns, complex fuel-air mixing parameters, and firing sequence variations. These factors may affect the effectiveness of nanoparticles in improving efficiency and emission reduction. Studying biodiesel formulations in multi-cylinder engines will accurately reflect actual performance conditions, optimizing biodiesel products for commercial and heavy-duty markets.
5. ACKNOWLEDGMENTS
The authors extend their appreciation to Taif University, Saudi Arabia, for supporting this work through project number (TU-DSPP-2024-91). This research was funded by Taif University, Saudi Arabia, Project N0. (TU-DSPP-2024-91).
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DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Publication Dates
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Publication in this collection
16 June 2025 -
Date of issue
2025
History
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Received
28 Dec 2024 -
Accepted
23 May 2025
