Introduction

As global energy demands surge, the overreliance on finite fossil fuel reserves has intensified concerns about their depletion and the environmental toll of their continued use. In response, this study focuses on the urgent need to identify and implement cleaner, renewable energy alternatives—particularly biofuels1,2. Biofuels come from renewable sources like crops and agricultural waste, unlike fossil fuels. This makes them sustainable and better for the environment. One of the key environmental advantages of biofuels lies in their carbon neutrality: the carbon dioxide released during combustion is largely offset by the CO₂ absorbed by the biomass during its growth phase. This balance significantly mitigates greenhouse gas emissions3,4. In addition to their environmental benefits, biofuels offer strategic advantages by diversifying the energy portfolio, reducing dependence on imported fuels, and stimulating rural economies through agricultural and processing activities5. As the world shifts toward sustainable energy, biofuels are emerging as pivotal players due to their cleaner combustion and compatibility with existing engine technologies. With increasing evidence linking diesel exhaust to adverse health effects, there is a pressing global imperative to produce cleaner-burning, locally sourced, and industrially viable fuels. Biodiesel, derived from plant oils or animal fats, is one such promising alternative to conventional diesel. It not only reduces air pollutants but also aligns with global efforts to curb climate change6,7. Though the concept of biofuels is not new—their use and potential have been explored for over a century—recent technological advances and environmental urgency have reignited research interest. This work contributes to the global momentum aimed at developing sustainable fuel solutions that address energy security, environmental health, and economic resilience, ultimately helping build a cleaner, greener future.

Numerous studies have demonstrated the potential of combining biodiesel with nanoparticle additives to enhance combustion efficiency, reduce fuel consumption, and lower emissions in compression ignition (CI) engines. Bikkavolu et al.8 assessed a B30 blend of Yellow Oleander Methyl Ester with carbon nanotubes (60 mg/L) and 10% Dimethyl Carbonate, achieving improved combustion, fuel economy, and reduced emissions within ASTM standards. Similarly, Pullagura et al.9 used SiO₂ nanoparticles in a ternary blend of 75% diesel, 15% SMME, and 10% iso-butanol, leading to increases in brake thermal efficiency (BTE), in-cylinder pressure (ICP), and net heat release rate (NHRR), while reducing brake-specific fuel consumption (BSFC) and emissions. Chinnasamy et al.10 tested aluminum oxide nanoparticles with pyrolyzed jatropha oil and noted pollutant reductions at 20% and 40% blending ratios. Dhahad et al.11 added ZnO nanoparticles to diesel and observed a 7% drop in fuel consumption and a 5% increase in thermal efficiency, though NOx emissions rose by 9%. Khan et al.12 reinforced this trend in a broader review, attributing improved emissions to enhanced oxygen availability from metal oxide nanoparticles. Gahavane et al.13 found CaO additives in soybean biodiesel improved thermal efficiency and reduced BSFC, despite a moderate NOx increase, while Mousavi and Heris14 showed ZnO improved diesel’s thermophysical and lubrication properties by lowering viscosity and friction.

Kumar et al.15 observed that ZnO-doped lemongrass biodiesel, especially at B30 blend levels, enhanced combustion and reduced exhaust emissions. Panithasan et al.16 used rice husk nanoparticles in jatropha biodiesel (J20 RH0.2), achieving a 4.21% increase in BTE and an 18.32% CO reduction. Ramachander et al.17 evaluated a ternary blend of pentanol, mahua, and diesel with 80 ppm SiO₂, resulting in better thermal efficiency and lower CO and HC emissions. Yadav et al.18 demonstrated that graphene oxide (40 ppm) in B20 blends reduced HC by 17.38%, CO by 36.41%, and NOx by 13%. Singh et al.19 achieved a 19.74% boost in BTE and a 13.79% drop in BSFC using carbon nanotubes in biodiesel blends, along with improved emissions. Musthafa et al.20 enhanced palm oil biodiesel with di-tert-butyl peroxide, resulting in a 6.7% BTE increase and lower HC, CO, and smoke levels. Demir et al.21 combined graphene and HHO gas, reporting improved efficiency and lower HC, CO, and PM emissions, although NOx slightly increased due to more complete combustion. Ozer et al.22 used borax decahydrate as a diesel additive, leading to improved combustion, reduced knocking, and better overall engine performance. Thiruselvam et al.23 studied the effects of CeO₂ nanoparticles (30–60 ppm) in palm biodiesel and observed significant improvements in BSFC (reduction by 5.71%–9.85%) and BTE (increased by 1.06%–1.61%), alongside reduced CO, HC, and NOx emissions. In a separate study, the same authors integrated CeO₂ with thermal barrier coatings (TBCs), which reduced fuel consumption by up to 20.57% and mitigated NOx increases commonly seen with TBCs24. They also introduced hydrogen-air induction into CeO₂-doped biodiesel, which further reduced specific fuel consumption by 20.67%, increased BTE by 6.28%, and significantly lowered CO, HC, smoke, and NOx emissions25.

Building on this momentum, Demir et al.26 incorporated urea into diesel, noting its NOx-reducing effect and thermal efficiency gains, similar to selective catalytic reduction systems. Chen et al.27 tested diesel blends with aluminum oxide, carbon nanotubes, and silicon dioxide nanoparticles using a YANMAR TF120M engine. Their study showed up to a 19.8% drop in BSFC and an 18.8% rise in BTE, with CNTs particularly effective at lowering NOx emissions. Jain et al.28 studied how alumina particle sizes influenced emissions and fuel efficiency. Using 50–100 ppm Al₂O₃, they reported BSFC reductions of 16.8%, NOx reductions of 25.1%, HC by 27.4%, and smoke by 14.8%. Sarma et al.29 added TiO₂ nanoparticles to a 20% mahua biodiesel and 80% diesel blend, resulting in CO, HC, and NOx reductions of 46.54%, 22.54%, and 2.3%, respectively. Kegl et al.30 provided a comprehensive review of nanomaterials’ dual function in enhancing combustion and raising potential environmental or health concerns. Jayaprabakar et al.31 synthesized CaO nanoparticles from biowaste and added them to waste cooking oil biodiesel. Engine tests revealed increased cylinder pressure and heat release rates at concentrations from 25 to 125 ppm. In a subsequent study, Sudarsanam and Jayaprabakar32 analyzed the combination of alumina and CaO nanoparticles (90 ppm total) with B20 WCO biodiesel in a CRDI engine. Their results showed up to 11% improvement in BSFC and significant emission reductions—CO by 43% and HC by 40%. Nagarajan and Balasubramanian33 further advanced this concept with a ternary blend of neem biodiesel, calcium oxide nanofluid, and a surfactant. Their B20 + NF configuration achieved a 35% BSFC reduction, a 38.91% improvement in BTE, and substantial cuts in HC (36.36%), CO (33.33%), NOx (49.35%), and smoke (25.13%). Livingston et al.34 incorporated 50 ppm CeO₂ into a blend of 20% pyrolysis oil and 80% diesel, recording higher BTE and CO and HC reductions of approximately 30% and 6%, respectively. Finally, Sudalaiyandi et al.35 evaluated nanoparticle-enhanced blends of rubber seed and linseed oil biodiesels. A 10% total biodiesel blend showed the best thermal efficiency and reduced CO and NO₂ levels compared to neat diesel, demonstrating promise for cleaner combustion even at lower biodiesel ratios.

Replacing conventional fossil fuels with bio-based alternatives offers a promising route to curbing greenhouse gas emissions, largely because the carbon dioxide absorbed during biomass cultivation helps offset emissions released during combustion. This cyclical carbon exchange makes biofuels a greener energy option. Additionally, the biofuel industry has the potential to invigorate local economies by creating job opportunities in both agricultural and energy sectors, particularly through the use of agricultural residues and organic waste materials—thereby fostering rural development. Ongoing research is focused on making biofuels more efficient and sustainable. Emerging approaches include enhancing biodiesel with nano-scale additives to overcome current limitations in fuel performance and emissions control. Although numerous studies have explored alternative fuels, the combined impact of multi-fuel strategies and nanotechnology-enhanced biofuels in CI engines remains an evolving field. A gap identified in the literature reveals a lack of focus on the application of sunflower biodiesel enriched with alumina nanoparticles. While prior works have explored alumina-enhanced biodiesels, this study is distinctly novel in its selection of sunflower oil methyl ester (SOME) as the base feedstock—an underutilized alternative with promising fuel properties. The biodiesel was combined with conventional diesel at the following ratios: 20% biodiesel to 80% diesel, 40% biodiesel to 60% diesel, 60% biodiesel to 40% diesel, 80% biodiesel to 20% diesel, and 100% biodiesel to 0% diesel. To further improve fuel stability and curb NOx emissions, antioxidant additives were also considered essential. The incorporation of alumina (Al₂O₃) as a nano additive played a central role in enhancing combustion efficiency and environmental performance. The main aim of this study is to evaluate the performance, combustion, and emission characteristics of a CI engine fueled with various SOME–diesel blends, with particular focus on a 40% blend (SOMED40) enhanced with Al₂O₃ nanoparticles, to determine its viability as a cleaner and more efficient diesel substitute. Furthermore, this work adopts a more comprehensive experimental framework than previous studies, integrating fuel property characterization, ultrasonic-assisted nanoparticle dispersion to ensure stability, and detailed combustion and emission diagnostics. Through this systematic approach, valuable insights were gained into optimizing CI engine output by tailoring fuel composition using renewable sources and nanotechnology.

Materials and methods

Formulation of fuel blends using biodiesel

The present investigation assesses the potential of sunflower seeds as a viable feedstock for biodiesel synthesis, driven by their abundant availability and favorable oil-yielding properties. Widely cultivated across several Indian states, sunflower seeds yield substantial oil quantities, positioning them as a promising candidate for renewable fuel development. Sunflower seeds used in this study were obtained from Vinayak Seeds Corporation, based in Ahmedabad, Gujarat, India. The seeds were processed using a mechanical oil expeller with a capacity of 30 kg/hour. Each processing cycle handled 5 kg of seeds, with oil extraction occurring through five successive passes to ensure maximum yield. The extracted oil was then subjected to filtration and allowed to settle in a glass reagent container for 10–12 h to eliminate residual particulates. The clarified oil was subsequently transferred to a clean glass beaker for the next phase. To make SOME, 1000 ml of sunflower seed oil was first heated. In a separate container, a mixture of 250 g of methanol and 10 g of KOH (purchased from Merck India Pvt. Ltd.) was prepared. This solution was gradually added to a round-bottom flask while stirring constantly. The mixture was kept under continuous stirring at 600 rpm and maintained at a temperature of 65 °C for one hour. Once the reaction was complete, the mixture was poured into a separatory funnel, where the biodiesel (upper layer) separated from the glycerol (bottom layer). To ensure proper separation, the liquid was moved to a second separatory funnel and left to stand for 24 h. This allowed for the removal of excess alcohol and acidic impurities. The resulting SOME share several characteristics with conventional diesel, including favorable combustion properties, biodegradability, non-toxicity, and renewability. Post-transesterification, the biodiesel exhibited enhanced spray behavior, lower viscosity and density, and a higher heating value, aligning it well with clean energy standards. Figure 1 illustrates the biodiesel production process.

Fig. 1
Fig. 1
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Flowchart of biodiesel production process from sunflower seeds.

Characterization of alumina nanoparticle

Al₂O₃ nanoparticles employed in the present study were procured from Reinste and Sisco Research Laboratories, India. The key physicochemical properties of the nanoparticles are summarized in Table 1. The crystalline structure of the alumina nanoparticles was confirmed using X-ray diffraction (XRD), as shown in Fig. 2. The presence of sharp and well-defined diffraction peaks at 2θ values of approximately 16.1°, 27.3°, 34°, 42.8°, 55.2°, 59°, and 66.8° corresponds to the (111), (220), (311), (400), (422), (511), and (440) crystallographic planes of α-phase alumina. These peaks closely match the Joint Committee on Powder Diffraction Standards (JCPDS) reference data, confirming the phase purity and high crystallinity of the nanoparticles. The observed peak broadening indicates nanoscale crystallite dimensions, which are desirable for catalytic and thermal enhancement during combustion. Scanning electron microscopy (SEM), presented in Fig. 3, reveals nearly spherical alumina nanoparticles with relatively uniform size distribution and limited agglomeration. Such morphology and high surface-area-to-volume ratio are advantageous for promoting enhanced heat transfer, catalytic oxidation, and improved combustion kinetics when dispersed in fuel.

Prior to nanoparticle addition, sunflower oil methyl ester (SOME) was blended with diesel at different volumetric ratios—SOMED20, SOMED40, SOMED60, SOMED80, and SOMED100—to identify a base blend that offers a balanced compromise between fuel properties, combustion stability, and engine compatibility. Among these blends, SOMED40 was selected for nanoparticle enrichment based on a comparative assessment of fuel properties and preliminary engine performance trends. SOMED40 demonstrated a favorable balance of viscosity, density, calorific value, volatility, and oxygen availability compared to higher biodiesel blends, which exhibited increased viscosity and reduced atomization quality. Lower blends (e.g., SOMED20), although closer to diesel in physical properties, showed comparatively weaker emission reduction potential. Thus, SOMED40 emerged as an optimal mid-range blend capable of maintaining stable combustion while maximizing the benefits of biodiesel oxygenation, making it a suitable candidate for further enhancement using nano-additives.

The concentration of alumina nanoparticles was fixed at 50 ppm based on combustion efficiency, dispersion stability, and agglomeration considerations. Previous studies on metal oxide nanoparticle-doped biodiesel fuels have consistently reported that concentrations in the range of 40–60 ppm provide optimal catalytic activity and thermal conductivity enhancement without causing particle clustering, injector fouling, or fuel instability18. At concentrations beyond this range, nanoparticle agglomeration becomes more pronounced, negatively affecting spray characteristics and combustion repeatability. Conversely, concentrations below this threshold may not deliver sufficient catalytic or thermal benefits. Therefore, 50 ppm was selected as an optimal and practically viable dosage that balances combustion enhancement with long-term dispersion stability.

For the preparation of the nano-enhanced fuel blend (SOMED40/Al₂O₃), the required quantity of alumina nanoparticles was first dispersed into the SOMED40 base fuel using mechanical stirring for 30 min. This was followed by ultrasonication at a frequency of 40 kHz for 45 min to ensure uniform nanoparticle dispersion and to minimize agglomeration. To enhance colloidal stability, 0.1% (v/v) Span 80 surfactant, procured from Sigma-Aldrich, India, was added to ensure uniform nanoparticle dispersion and to maintain suspension stability during storage and engine testing. This combined mechanical–ultrasonic dispersion approach ensured homogeneous distribution of nanoparticles within the fuel matrix, thereby enabling consistent catalytic and thermal effects during combustion.

Table 1 Essential physicochemical properties of alumina nanoparticles.
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Fig. 2
Fig. 2
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XRD pattern of aluminium oxide nanoparticles.

Fig. 3
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SEM micrograph of alumina nanoparticles.

Stability analysis of Al₂O₃-dispersed SOME

The stability of Al₂O₃ nanoparticles dispersed in sunflower oil methyl ester (SOME) plays a decisive role in ensuring uniform fuel properties, consistent atomization, and reliable combustion behavior during engine operation. To assess the colloidal stability of the nano-enhanced fuel, zeta-potential analysis was carried out, as it provides a direct indication of interparticle electrostatic interactions and resistance to agglomeration. Figure 4 illustrates the zeta-potential distribution of Al₂O₃ nanoparticles suspended in the SOME matrix. The distribution is predominantly centered around − 42 mV, with values ranging approximately between − 50 mV and − 35 mV. This relatively narrow distribution and strong negative surface charge indicate a well-dispersed and electrostatically stable nanofluid system. In colloidal science, absolute zeta-potential values exceeding ± 30 mV are widely accepted as indicative of good dispersion stability, as strong repulsive forces prevent particle agglomeration and sedimentation. The observed negative zeta-potential suggests that Al₂O₃ nanoparticles acquire sufficient surface charge in the SOME medium, leading to enhanced electrostatic repulsion between particles. This behavior effectively suppresses clustering and ensures that the nanoparticles remain uniformly suspended over time. Such stability is particularly important for nano-additive fuels, as particle agglomeration can adversely affect injector performance, fuel spray characteristics, and combustion repeatability. The stable dispersion confirmed by the zeta-potential results supports the suitability of the Al₂O₃-enhanced SOME blend for engine testing. Consistent nanoparticle suspension ensures uniform catalytic and thermal effects during combustion, thereby improving the reliability and reproducibility of the performance, combustion, and emission results reported in this study.

Fig. 4
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Zeta-potential distribution of Al₂O₃ nanoparticles dispersed in sunflower oil methyl ester.

Properties of fuel

Table 2 showcases the key fuel characteristics for various diesel–biodiesel mixtures. As the proportion of SOME increases from 20% to 100%, a consistent upward trend is noted in parameters such as density, calorific value, kinematic viscosity, and flashpoint. The density rises with higher biodiesel content due to its inherently denser nature compared to diesel. Similarly, the kinematic viscosity grows, reflecting biodiesel’s thicker consistency, which can influence spray and atomization patterns during combustion. Notably, the gross calorific value sees a significant enhancement in the SOMED40/Al2O3, indicating improved energy output. The flashpoint also climbs with greater biodiesel content, attributed to biodiesel’s naturally higher ignition temperature, which contributes to safer storage and transport conditions36.

Table 2 The essential fuel properties for different diesel–biodiesel blends.
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Experimental endeavor

A comprehensive experimental study was carried out using a four-stroke, liquid-cooled, single-cylinder diesel engine with a direct injection system. The experimental setup, shown in Fig. 5, was equipped with a variety of sensors and high-precision instruments to allow for real-time data collection. The system included an eddy current dynamometer for load application, a computerized data acquisition system, fuel and lubrication subsystems, and a water-based cooling mechanism, all ensuring accurate measurements of cylinder pressure, air flow, and fuel flow rates. The engine’s specifications are detailed in Table 3. Tests were performed at four distinct load levels—0%, 25%, 50%, and 75%—using the eddy current dynamometer, with the engine speed maintained at a constant 1500 rpm. Emission parameters such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) were measured using an AVL Five Gas Analyzer, as described in Table 4. The ARAI-EDACS controller system was employed to control engine operation, monitor temperatures, and automatically record emission data to a connected computer. Baseline data was initially gathered by operating the engine on conventional diesel fuel for 30 min under low load conditions until the system reached thermal and operational stability. Full-load measurements were then recorded. The study was subsequently expanded to include testing of various fuel blends, including SOMED20, SOMED40, SOMED60, SOMED80, SOMED100, and SOMED40/Al2O3, all evaluated under full-load conditions. Each test was repeated three times to ensure the consistency and accuracy of the results for further analysis.

Fig. 5
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(a) Original photograph of the experimental setup; (b) schematic representation of the experimental process showing (1) engine, (2) loading unit, (3) air tank, (4) fuel tank, (5) data processing CPU, (6) display unit, (7) AVL gas analyzer, (8) sensor connections, and (9) exhaust system.

Table 3 Details of the test engine’s technical features.
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Table 4 Details of the exhaust gas analyzer specifications.
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Uncertainty analysis

To enhance the reliability and transparency of the experimental findings, a detailed uncertainty analysis was conducted using Holman’s method of error propagation. This approach accounts for the combined influence of individual instrument uncertainties on calculated parameters through the root-sum-square (RSS) method37,38. The general expression for the combined uncertainty of a derived quantity R is given by:

$$\:{U}_{R}=\:\sqrt{{\left(\frac{\partial\:R}{\partial\:{x}_{1}}\:{U}_{{x}_{1}}\right)}^{2}+\:{\left(\frac{\partial\:R}{\partial\:{x}_{2}}\:{U}_{{x}_{2}}\right)}^{2}+{\cdots\:+\left(\frac{\partial\:R}{\partial\:{x}_{n}}\:{U}_{{x}_{n}}\right)}^{2}\:}$$
(1)

where:

  • \(\:{U}_{R}\:\) = Combined uncertainty of the result R.

  • \(\:{U}_{{x}_{n}}\) = Uncertainty of the input variable xn.

  • \(\:\frac{\partial\:R}{\partial\:{x}_{n}}\:\) = Sensitivity coefficient of R with respect to xn.

This method was applied to compute uncertainties in critical output parameters such as BTE, BSFC, and major emission characteristics (CO, HC, NOx, smoke opacity).

Instrumentation uncertainty

The uncertainties of all primary measuring instruments were obtained from manufacturer specifications and calibration records. These uncertainties represent the maximum possible deviation in each measured quantity and were used directly in the error propagation analysis. Table 5 summarizes the individual uncertainties associated with each measuring instrument used in the experiments.

Table 5 Measurement uncertainties in experimental setup.
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Combined uncertainty Estimation

The combined uncertainty for selected derived parameters was calculated using the above RSS method. Key expressions used include:

$$\:Break\:Power\:\left(BP\right)=\:\frac{2\pi\:NT}{60}$$
(2)

where N is engine speed and T is brake torque.

$$\:BSFC=\:\frac{{m}_{f}}{BP}$$
(3)
$$\:BTE=\:\frac{BP}{{m}_{f}\times\:CV}$$
(4)

where \(\:{m}_{f}\:\)is the fuel mass flow rate and CV is the fuel calorific value.

Based on error propagation through these formulas, the combined uncertainties for key output parameters were found to be as detailed in Table 6.

Table 6 Combined uncertainty values for key performance and emission parameters.
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These results confirm that the uncertainty levels lie within acceptable experimental tolerances and support the validity of the performance and emission data presented.

Results and discussion

Analysis of performance characteristics

Brake-specific fuel consumption

Brake-specific fuel consumption is a direct indicator of fuel utilization efficiency and combustion effectiveness. As observed in Fig. 6, BSFC decreases with increasing engine load for all tested fuels, which is consistent with conventional CI engine behaviour due to improved combustion efficiency and reduced relative heat losses at higher loads. A similar load-dependent BSFC reduction has been widely reported for biodiesel and nano-enhanced fuels in the literature27,28. Among the biodiesel blends, SOMED40 exhibited lower BSFC than higher biodiesel fractions (SOMED60–SOMED100), primarily due to its balanced viscosity and improved atomization compared to higher blends. Quantitatively, at full load, SOMED40 recorded a BSFC of 0.388 kg/kWh, which is comparable to values reported for B30–B40 biodiesel blends in earlier studies using alumina and ceria nanoparticles, where BSFC reductions of 1–3% were observed relative to neat biodiesel23,28.

The addition of 50 ppm Al₂O₃ nanoparticles to SOMED40 further reduced BSFC by 1.55% compared to the base SOMED40 blend. This magnitude of improvement closely aligns with the findings of Chen et al.27, who reported a BSFC reduction of approximately 2% for alumina-dispersed diesel blends, and Jain et al.28, who observed BSFC reductions in the range of 1.8–2.5% using 50–100 ppm Al₂O₃ nanoparticles. The reduction in BSFC is attributed to the catalytic action of alumina nanoparticles, which enhances oxidation kinetics, shortens ignition delay, and improves heat transfer within the combustion chamber. The high surface area of Al₂O₃ nanoparticles promotes secondary atomization and faster evaporation of fuel droplets, resulting in more complete combustion and lower fuel consumption for a given power output. Overall, the BSFC trends observed in this study are in strong agreement with previously reported biodiesel–nanoparticle investigations, confirming that alumina nanoparticles effectively mitigate the inherent fuel economy penalty associated with biodiesel fuels.

Fig. 6
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Variation of BSFC with load for the test fuels.

Brake thermal efficiency

Brake thermal efficiency represents the effectiveness of converting the chemical energy of fuel into useful mechanical work. As shown in Fig. 7, BTE increased with engine load for all tested fuels, which is a well-established trend due to improved combustion temperature and reduced relative heat losses at higher loads. Similar BTE trends have been consistently reported in nano-additive biodiesel studies21,27,30. Among the base blends, SOMED40 exhibited slightly higher BTE than SOMED20 and significantly higher BTE than SOMED60–SOMED100. This behaviour is attributed to the optimal compromise between oxygen availability from biodiesel and favourable spray characteristics retained from diesel. At full load, SOMED40 achieved a BTE of 24.11%, which compares favourably with reported BTE values of 23–25% for mid-range biodiesel blends in earlier studies16,23.

A notable enhancement was observed with the SOMED40/Al₂O₃ blend, which delivered a peak BTE of 25.4%—representing a 5.35% improvement over neat SOMED40. This improvement is quantitatively comparable to the 4–6% BTE enhancement reported by Thiruselvam et al.23 using CeO₂ nanoparticles and the 5.1% improvement reported by Dhahad and Chaichan11 using Al₂O₃-doped diesel fuel. The superior thermal efficiency achieved with alumina addition is attributed to its dual role as a combustion catalyst and thermal conductivity enhancer. Alumina nanoparticles facilitate faster heat diffusion, promote micro-scale turbulence, and improve the oxidation of fuel-rich zones, thereby increasing the effective pressure rise during the combustion phase. Importantly, the BTE achieved with SOMED40/Al₂O₃ approaches that of neat diesel (28.16%), demonstrating that nano-additive enrichment can significantly bridge the efficiency gap between biodiesel and conventional diesel fuels. This finding reinforces previous conclusions that metal oxide nanoparticles are effective in enhancing biodiesel combustion efficiency without requiring engine modifications27,30.

Fig. 7
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Effect of load on the BTE of test fuels.

Exhaust gas temperature

Exhaust gas temperature (EGT) is a reliable indicator of combustion intensity, heat release behaviour, and post-combustion energy distribution in CI engines. As illustrated in Fig. 8, EGT increased progressively with engine load for all tested fuels, which is a typical trend attributed to increased fuel injection, higher in-cylinder temperatures, and enhanced oxidation rates at elevated loads. Compared to neat diesel, all SOME blends exhibited higher EGT values throughout the load range. At full load, diesel recorded an EGT of approximately 229 °C, while SOMED40 reached about 243 °C, corresponding to an increase of nearly 6%. Similar EGT elevations (5–8%) for biodiesel–diesel blends have been reported in earlier studies and are mainly associated with the inherent oxygen content of biodiesel and prolonged diffusion combustion phases16,17.

The incorporation of alumina nanoparticles into SOMED40 resulted in a further rise in EGT, with SOMED40/Al₂O₃ exhibiting an EGT of around 259 °C at full load. This represents an increase of approximately 6.6% compared to SOMED40 and about 13% relative to diesel. These results are in close agreement with the findings of Chen et al.27, who reported a 10–15% increase in EGT for Al₂O₃- and CNT-enhanced diesel fuels, and Jain et al.28, who observed EGT increments of 8–12% using 50–100 ppm alumina nanoparticles in biodiesel blends. The elevated EGT observed in the present study indicates intensified and more complete combustion rather than excessive heat loss, as corroborated by the simultaneous improvement in brake thermal efficiency and reduction in BSFC.

From a mechanistic perspective, alumina nanoparticles enhance EGT through their catalytic activity and high thermal conductivity. The nanoparticles promote faster oxidation of fuel-rich zones, improve secondary atomization, and facilitate more uniform heat distribution inside the combustion chamber, leading to higher exhaust enthalpy. Similar combustion-enhancing effects of metal oxide nanoparticles—such as CeO₂, TiO₂, and ZnO—on EGT have been widely reported in biodiesel-fueled CI engines23,29,30. Importantly, the moderate increase in EGT observed for SOMED40/Al₂O₃ aligns with recent literature, which suggests that such behaviour is indicative of improved combustion efficiency and reduced incomplete combustion losses rather than detrimental thermal penalties30.

Fig. 8
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Variation of exhaust gas temperature with load for various fuels.

Analysis of combustion characteristics

Net heat release rate

The net heat release rate (HRR) provides direct insight into combustion phasing, ignition delay, and the intensity of premixed and diffusion-controlled combustion stages. Figure 9 illustrates the variation of HRR with crank angle for diesel, SOMED blends, and the alumina nanoparticle-enhanced SOMED40/Al₂O₃ fuel. For all test fuels, the peak HRR occurred close to top dead center (TDC), indicating favourable combustion phasing. Diesel exhibited the highest peak HRR of approximately 61.1 J/°CA due to its superior volatility and faster premixed combustion. In comparison, SOMED40 recorded a peak HRR of about 50.1 J/°CA, reflecting the relatively longer ignition delay and slower vaporization associated with biodiesel blends. Similar reductions in HRR for biodiesel–diesel blends have been reported by Chen et al. and Panithasan et al., who attributed this behaviour to higher viscosity and reduced premixed combustion fractions in biodiesel fuels16,27. The addition of alumina nanoparticles to SOMED40 significantly enhanced the combustion intensity, increasing the peak HRR to approximately 56.0 J/°CA—an improvement of nearly 11.7% over neat SOMED40. This enhancement is quantitatively comparable to previous investigations using metal oxide nanoparticles. Thiruselvam et al.23 observed notable increases in peak HRR with CeO₂ nanoparticle addition to biodiesel. The improved HRR in the present study indicates accelerated combustion kinetics, reduced ignition delay, and enhanced premixed combustion due to the catalytic action of alumina nanoparticles. The nanoparticles promote faster oxidation reactions and improved air–fuel mixing, thereby intensifying the early combustion phase.

From a mechanistic standpoint, alumina nanoparticles contribute to higher HRR through their high surface area and thermal conductivity, which enhance fuel evaporation and heat transfer within the combustion chamber. The nanoparticles act as active catalytic sites, facilitating rapid decomposition of fuel molecules and improving local turbulence, which strengthens the premixed combustion phase. Similar HRR enhancement mechanisms have been reported for CeO₂, TiO₂, and ZnO nanoparticles in biodiesel-fueled CI engines29,30. Importantly, the increased HRR observed for SOMED40/Al₂O₃ did not lead to excessive pressure rise or combustion instability, indicating controlled and efficient combustion. These findings confirm that alumina nanoparticles effectively compensate for the inherent combustion limitations of biodiesel blends and align well with trends reported in recent biodiesel–nanoparticle literature.

Fig. 9
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Effect of crank angle on net heat release rate.

Combustion pressure

In-cylinder combustion pressure is a key indicator of combustion quality, ignition delay, and the rate of energy release during the premixed and diffusion phases. Figure 10 illustrates the variation of cylinder pressure with crank angle for diesel, SOMED blends, and the alumina nanoparticle-enhanced SOMED40/Al₂O₃ fuel. For all fuels, the peak pressure occurred slightly after top dead center, indicating stable and well-phased combustion. Diesel exhibited the highest peak cylinder pressure of approximately 60.5 bar, which is attributed to its superior volatility, shorter ignition delay, and rapid premixed combustion. In contrast, SOMED40 showed a lower peak pressure of about 55.5 bar, reflecting the slower vaporization and longer ignition delay associated with biodiesel blends. Similar reductions in peak pressure for biodiesel–diesel blends have been consistently reported in the literature and are primarily linked to higher viscosity and lower calorific value of biodiesel fuels16,27.

The addition of alumina nanoparticles to SOMED40 resulted in a noticeable increase in peak cylinder pressure, reaching approximately 57.4 bar—representing an improvement of about 3.4% over neat SOMED40. This enhancement is in good agreement with previously reported studies on metal oxide nanoparticle additives. Chen et al.27 reported increases of 3–6% in peak cylinder pressure for Al₂O₃- and CNT-enhanced fuels, while Jain et al.28 observed similar pressure improvements with alumina nanoparticles in the range of 50–100 ppm. The increase in pressure indicates improved premixed combustion intensity and faster heat release, which can be attributed to the catalytic role of alumina nanoparticles in accelerating oxidation reactions and improving fuel–air mixing.

Actually, alumina nanoparticles enhance cylinder pressure by reducing ignition delay and promoting rapid energy release during the early stages of combustion. Their high thermal conductivity facilitates faster heat transfer within the combustion chamber, while their large surface area provides active sites for catalytic oxidation of fuel molecules. These effects lead to a more intense premixed combustion phase, resulting in higher pressure rise without inducing abnormal combustion or knocking. Similar pressure enhancement mechanisms have been reported for CeO₂, TiO₂, and ZnO nanoparticle-assisted biodiesel combustion23,29,30. Importantly, the pressure rise observed for SOMED40/Al₂O₃ remained lower than that of neat diesel, confirming controlled combustion behaviour while still achieving significant improvement over the base biodiesel blend.

Fig. 10
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Effect of crank angle on combustion pressure.

Analysis of emission characteristics

CO emission

Carbon monoxide (CO) emissions arise primarily from incomplete oxidation of carbonaceous species and are strongly affected by fuel oxygen content, combustion temperature, and local air–fuel equivalence ratio. As shown in Fig. 11, CO emissions decreased with increasing engine load for all tested fuels due to enhanced in-cylinder temperature and improved oxidation efficiency. Compared to diesel, all SOME blends exhibited lower CO emissions throughout the load range. At full load, diesel emitted approximately 0.095% CO, whereas SOMED40 produced about 0.085%, corresponding to a reduction of nearly 10%. Similar CO reductions for mid-range biodiesel blends have been reported by Kumar et al.15 using ZnO-enhanced lemongrass biodiesel and by Panithasan et al.16 using rice husk nanoparticle–assisted biodiesel, both attributing the trend to biodiesel’s inherent oxygen content facilitating more complete combustion.

A substantial further reduction in CO emissions was achieved with the addition of alumina nanoparticles. At full load, the SOMED40/Al₂O₃ blend recorded a CO emission of approximately 0.065%, representing a reduction of about 23.5% compared to neat SOMED40 and nearly 32% relative to diesel. This magnitude of reduction is consistent with several recent biodiesel–nanoparticle studies employing different nano-additives. Yadav et al.18 reported CO reductions of over 35% using graphene oxide nanoparticles in biodiesel blends, while Singh et al.19 observed CO reductions of approximately 20–30% with carbon nanotube-dispersed biodiesel fuels. Similarly, Nagarajan and Balasubramanian33 demonstrated significant CO mitigation using calcium oxide nanofluids in biodiesel–diesel ternary blends. These studies collectively confirm that nanoparticle-assisted fuels consistently outperform neat biodiesel in suppressing CO formation.

The reduction in CO emissions with alumina addition can be attributed to its catalytic oxidation capability and enhancement of combustion homogeneity. Al₂O₃ nanoparticles provide high-surface-area active sites that promote the conversion of CO to CO₂, particularly in fuel-rich regions near the spray core. In addition, nanoparticles improve secondary atomization and micro-scale turbulence, resulting in better air–fuel mixing and reduced quenching zones. Similar catalytic mechanisms have been widely reported for metal oxide nanoparticles such as ZnO, TiO₂, and CaO in biodiesel-fueled CI engines11,29,33. The consistently lower CO emissions observed with SOMED40/Al₂O₃ therefore confirm that alumina nanoparticles play a decisive role in enhancing combustion completeness and align well with broader findings in recent biodiesel–nanoparticle research.

Fig. 11
Fig. 11
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Variation of CO with load for multiple test fuels.

HC emission

Unburned hydrocarbon (HC) emissions originate from incomplete combustion, fuel wall impingement, flame quenching near cold surfaces, and fuel trapped in crevices. Figure 12 shows the variation of HC emissions with engine load for diesel, SOMED blends, and the SOMED40/Al₂O₃ fuel. For all fuels, HC emissions increased slightly with load due to higher fuel injection quantities; however, biodiesel blends consistently emitted lower HC than diesel across the operating range. At full load, diesel produced approximately 31 ppm of HC, while SOMED40 emitted around 27 ppm, corresponding to a reduction of nearly 13%. Similar HC reductions for biodiesel blends have been reported by Ramachander et al.15 and Kumar et al.17, who attributed the decrease to the inherent oxygen content of biodiesel, which enhances oxidation of unburned fuel fragments.

The incorporation of alumina nanoparticles into SOMED40 resulted in a further and more pronounced reduction in HC emissions. At full load, the SOMED40/Al₂O₃ blend recorded an HC emission of approximately 23 ppm, representing a reduction of about 14.8% compared to neat SOMED40 and nearly 26% relative to diesel. These results are consistent with recent nanoparticle-assisted biodiesel studies employing different nano-additives. Yadav et al. reported HC reductions of 17–20% using graphene oxide nanoparticles18, while Singh et al.19 observed HC reductions exceeding 15% with carbon nanotube-enhanced biodiesel blends. Additionally, Sarma et al.29 demonstrated significant HC mitigation using TiO₂ nanoparticles in mahua biodiesel, confirming that nano-additives effectively suppress incomplete combustion products.

The enhanced HC reduction achieved with alumina nanoparticles can be attributed to their catalytic oxidation behaviour and ability to improve combustion homogeneity. Al₂O₃ nanoparticles promote oxidation of unburned hydrocarbons by providing active catalytic sites and increasing local combustion temperature in fuel-rich regions. Furthermore, improved atomization and secondary breakup of fuel droplets reduce wall wetting and fuel quenching, which are primary sources of HC emissions. Similar mechanisms have been reported for ZnO, TiO₂, and CaO nanoparticle-enhanced biodiesel fuels11,29,33. The substantial HC reduction observed with SOMED40/Al₂O₃ therefore confirms that alumina nanoparticles effectively enhance combustion completeness and aligns well with the broader biodiesel–nanoparticle emission literature.

Fig. 12
Fig. 12
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Variation of HC with load for multiple test fuels.

NOx emission

Nitrogen oxides (NOx) emissions in CI engines are predominantly governed by in-cylinder temperature, oxygen availability, pressure, and residence time of nitrogen at elevated temperatures. Figure 13 presents the variation of NOx emissions with engine load for diesel, SOMED blends, and the SOMED40/Al₂O₃ fuel. As expected, NOx emissions increased with load for all fuels due to higher combustion temperatures and intensified oxidation reactions. At full load, diesel produced approximately 490 ppm of NOx, whereas SOMED40 recorded about 440 ppm, corresponding to a reduction of nearly 10%. Similar NOx reductions for mid-range biodiesel blends have been reported by Musthafa and Asokan20 and Demir et al.21, who attributed the behaviour to lower peak flame temperatures and shorter residence time of nitrogen at high temperatures when biodiesel is blended with diesel.

The addition of alumina nanoparticles to SOMED40 resulted in a further reduction in NOx emissions, with SOMED40/Al₂O₃ recording approximately 415 ppm at full load—representing a decrease of about 13.3% compared to SOMED40 and nearly 25% relative to diesel. These results are consistent with recent biodiesel–nanoparticle studies employing different additive systems. Nagarajan and Balasubramanian33 reported NOx reductions exceeding 40% using calcium oxide nanofluids in biodiesel blends, while Livingston et al.34 observed notable NOx mitigation through nanoparticle-assisted combustion of bio-derived fuels. Although some nanoparticle additives have been reported to increase NOx due to enhanced combustion temperatures, the present results indicate that alumina nanoparticles effectively balance combustion intensification with thermal moderation.

Mechanistically, the reduction in NOx emissions with alumina addition is attributed to improved combustion uniformity and suppression of localized high-temperature zones. Al₂O₃ nanoparticles act as micro-scale heat sinks, absorbing and redistributing thermal energy, thereby reducing peak flame temperatures responsible for thermal NOx formation. Additionally, the nanoparticles promote faster combustion completion, reducing the residence time of nitrogen in high-temperature regions. Bikkavolu et al.6 also observed a notable reduction in NOx emissions (≈ 13.2%) for CNT-enhanced Y20 yellow oleander biodiesel blends. The reduction was associated with improved combustion uniformity, reduced ignition delay, and suppression of localized peak flame temperatures, indicating that nano-additives can enhance combustion efficiency while simultaneously moderating thermal NOx formation.

Fig. 13
Fig. 13
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Variation of NOx with load for various test fuels.

Smoke opacity

Smoke opacity is primarily associated with soot formation resulting from incomplete combustion, fuel-rich diffusion zones, and poor air–fuel mixing. Figure 14 shows the variation of smoke opacity with engine load for diesel, SOMED blends, and the SOMED40/Al₂O₃ fuel. For all test fuels, smoke opacity increased with engine load due to higher fuel injection quantities and richer combustion conditions. However, biodiesel blends consistently exhibited lower smoke opacity than diesel across the load range. At full load, diesel recorded a smoke opacity of approximately 0.00442 kg/m³, whereas SOMED40 produced about 0.00380 kg/m³, corresponding to a reduction of nearly 14%. Similar reductions in smoke emissions for biodiesel–diesel blends have been reported by Ramachander et al.17 and Singh et al.19, who attributed the trend to the oxygenated nature of biodiesel, which suppresses soot precursor formation and enhances oxidation of carbonaceous particulates.

A further and more pronounced reduction in smoke opacity was achieved with the addition of alumina nanoparticles. At full load, the SOMED40/Al₂O₃ blend exhibited a smoke opacity of approximately 0.00321 kg/m³, representing a reduction of about 15.8% compared to SOMED40 and nearly 27% relative to diesel. These values are consistent with recent nanoparticle-assisted biodiesel studies using different nano-additives. Yadav et al.18 reported smoke reductions exceeding 20% using graphene oxide nanoparticles, while Nagarajan and Balasubramanian33 observed reductions of up to 25% with calcium oxide nanofluids in biodiesel blends. Similar soot suppression trends have also been reported for TiO₂ and ZnO nanoparticle-enhanced biodiesel fuels11,29, confirming the effectiveness of nano-additives in particulate emission control.

The reduction in smoke opacity with alumina addition can be attributed to improved oxidation of soot precursors and enhanced combustion homogeneity. Al₂O₃ nanoparticles promote faster breakdown of long-chain hydrocarbons and inhibit the formation of fuel-rich pockets that favour soot nucleation. In addition, improved atomization and secondary breakup of fuel droplets reduce diffusion-controlled combustion regions, which are major sources of smoke formation. Similar soot-mitigating mechanisms have been reported for metal oxide nanoparticles such as CaO, TiO₂, and ZnO in biodiesel-fueled CI engines29,33,30. The substantial smoke reduction observed with SOMED40/Al₂O₃ therefore confirms that alumina nanoparticles play a crucial role in particulate emission mitigation, further strengthening the environmental viability of nano-enhanced biodiesel fuels.

Fig. 14
Fig. 14
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Variation of smoke opacity with load for various fuels.

Conclusion

This research highlights the potential of alumina nanoparticles to enhance the performance and emissions profile of sunflower oil methyl ester biodiesel, focusing specifically on a 40% biodiesel blend (SOMED40). Based on the experimental results, the following conclusions can be drawn:

  • The SOMED40 blend with 50 ppm of alumina nanoparticles exhibited the highest brake thermal efficiency among all biodiesel blends at full engine load, improving efficiency by 5.35% over the unmodified SOMED40 and approaching the performance level of conventional diesel.

  • This nanoparticle-enhanced blend also reduced BSFC by 1.55% compared to SOMED40, indicating improved fuel economy. Although the BSFC remained marginally higher than that of diesel, the improvement affirms the catalytic role of alumina nanoparticles in enhancing combustion characteristics.

  • Carbon monoxide emissions were consistently lower across all load levels with the SOMED40/Al₂O₃ blend, indicating more complete and efficient combustion.

  • Hydrocarbon emissions decreased substantially with the SOMED40/Al₂O₃ blend—by 14.81% compared to SOMED40 and by 25.80% compared to diesel—suggesting better oxidation of unburned fuel components.

  • NOx emissions, a typical concern with biodiesel fuels, were reduced by 13.33% compared to SOMED40 and by 25% relative to diesel. This reduction is attributed to improved ignition quality and moderated combustion facilitated by the nanoparticles.

  • Smoke opacity was also reduced with the SOMED40/Al₂O₃ blend, showing a decrease of 15.79% compared to SOMED40 and 27.27% compared to diesel, indicating a cleaner combustion process with fewer particulate emissions.

  • Higher exhaust gas temperatures were observed with the SOMED40/Al₂O₃ blend, reflecting more efficient energy release during combustion—likely due to enhanced atomization and thermal conductivity introduced by the alumina nanoparticles.

In summary, the incorporation of alumina nanoparticles into the SOMED40 blend substantially improved engine performance and reduced pollutant emissions, making it a strong candidate for sustainable diesel substitution. Moreover, the low cost, abundant availability, and ease of dispersion of Al₂O₃ nanoparticles—combined with the compatibility of the SOMED40/Al₂O₃ blend with existing diesel engines without requiring modifications—enhance its economic feasibility and scalability. These attributes underscore the blend’s commercial potential, particularly for decentralized and rural energy systems. Future investigations should focus on evaluating the long-term storage stability and aging behaviour of Al₂O₃-enhanced SOME blends under varying environmental conditions. Detailed injector durability and deposit formation studies are also recommended to assess the practical applicability of nano-additive fuels in prolonged engine operation. In addition, optimizing nanoparticle concentration and exploring hybrid nano-additives may further improve combustion efficiency and emission mitigation. Advanced diagnostic techniques, such as optical combustion analysis and detailed particulate characterization, could provide deeper insights into the underlying combustion mechanisms. Finally, comprehensive life-cycle assessment and techno-economic analysis are suggested to evaluate the environmental sustainability and commercial feasibility of large-scale implementation.